[BOLT][test] enable GNU extensions, use C++ compiler, remove unnecessary target (#117043)

1. With a Clang that doesn't default to GNU extensions they need to be enabled explicitly.
2. The X86 directory lit config sets it already, there's no reason for this test to do it by itself.
3. The C frontend executable will fail if there's for example a Clang resource file for the C++ mode that sets C++-specific options:
```
+ /home/tambre/dev/llvm/build/bin/clang --target=x86_64-unknown-linux-gnu -fPIE -fuse-ld=lld -Wl,--unresolved-symbols=ignore-all -pie -fPIC -shared /home/tambre/dev/llvm/bolt/test/R_ABS.pic.lld.cpp -o /home/tambre/dev/llvm/build/tools/bolt/test/Output/R_ABS.pic.lld.cpp.tmp.so -Wl,-q -fuse-ld=lld
clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]
error: invalid argument '-std=c23' not allowed with 'C++'
```

bolt/test/AArch64/data-at-0-offset.c

@@ -1,4 +1,4 @@
-// RUN: %clang %cflags -O2 -fPIE -Wl,-q -pie  %s -o %t.exe
+// RUN: %clang %cflags -O2 -fPIE -std=gnu99 -Wl,-q -pie  %s -o %t.exe
 // RUN: llvm-bolt %t.exe -o %t.bolt 2>&1 | FileCheck %s
 // CHECK-NOT: BOLT-WARNING: unable to disassemble instruction at offset
 

bolt/test/AArch64/double_jump.cpp

@@ -1,6 +1,6 @@
 // A contrived example to test the double jump removal peephole.
 
-// RUN: %clang %cflags -O0 %s -o %t.exe
+// RUN: %clangxx %cxxflags -O0 %s -o %t.exe
 // RUN: llvm-bolt %t.exe -o %t.bolt --peepholes=double-jumps | \
 // RUN:   FileCheck %s -check-prefix=CHECKBOLT
 // RUN: llvm-objdump --no-print-imm-hex -d %t.bolt | FileCheck %s

bolt/test/R_ABS.pic.lld.cpp

@@ -6,7 +6,7 @@
  * with libc available.
  * REQUIRES: system-linux
  *
- * RUN: %clang %cflags -fPIC -shared %s -o %t.so -Wl,-q -fuse-ld=lld
+ * RUN: %clangxx %cxxflags -fPIC -shared %s -o %t.so -Wl,-q -fuse-ld=lld
  * RUN: llvm-bolt %t.so -o %t.so.bolt --relocs
  */
 

bolt/test/X86/double-jump.test

@@ -4,7 +4,7 @@
 ## correctly on Windows e.g. subshell execution
 REQUIRES: shell
 
-RUN: %clang %cflags %p/Inputs/double_jump.cpp -o %t.exe
+RUN: %clangxx %cxxflags %p/Inputs/double_jump.cpp -o %t.exe
 RUN: (llvm-bolt %t.exe --peepholes=double-jumps \
 RUN:   --eliminate-unreachable -o %t 2>&1 \
 RUN:   && llvm-objdump -d %t --print-imm-hex --no-show-raw-insn) | FileCheck %s

bolt/test/X86/dwarf5-df-inlined-subroutine-gc-sections-range.test

@@ -6,7 +6,7 @@
 ; RUN: -split-dwarf-file=main.dwo -o main.o
 ; RUN: llvm-mc -dwarf-version=5 -filetype=obj -triple x86_64-unknown-linux %p/Inputs/dwarf5-df-inlined-subroutine-gc-sections-range-helper.s \
 ; RUN: -split-dwarf-file=helper.dwo -o helper.o
-; RUN: %clang --target=x86_64-pc-linux -fuse-ld=lld -Wl,-gc-sections -Wl,-q -gdwarf-5 -gsplit-dwarf=split main.o helper.o -o main.exe
+; RUN: %clang -fuse-ld=lld -Wl,-gc-sections -Wl,-q -gdwarf-5 -gsplit-dwarf=split main.o helper.o -o main.exe
 ; RUN: llvm-bolt main.exe -o main.exe.bolt --update-debug-sections
 ; RUN: llvm-dwarfdump --debug-addr main.exe > log.txt
 ; RUN: llvm-dwarfdump --debug-rnglists --verbose --show-form main.dwo >> log.txt

bolt/test/X86/jmp-optimization.test

@@ -4,7 +4,7 @@
 ## correctly on Windows e.g. unsupported parameter expansion
 REQUIRES: shell
 
-RUN: %clang %cflags -O2 %S/Inputs/jmp_opt{,2,3}.cpp -o %t
+RUN: %clangxx %cxxflags -O2 %S/Inputs/jmp_opt{,2,3}.cpp -o %t
 RUN: llvm-bolt -inline-small-functions %t -o %t.bolt
 RUN: llvm-objdump -d %t.bolt --print-imm-hex | FileCheck %s
 

bolt/test/X86/match-functions-with-call-graph.test

@@ -2,7 +2,7 @@
 
 # REQUIRES: system-linux
 # RUN: split-file %s %t
-# RUN: %clang %cflags %t/main.cpp -o %t.exe -Wl,-q -nostdlib
+# RUN: %clangxx %cxxflags %t/main.cpp -o %t.exe -Wl,-q -nostdlib
 # RUN: llvm-bolt %t.exe -o %t.out --data %t/yaml --profile-ignore-hash -v=1 \
 # RUN:   --dyno-stats --print-cfg --infer-stale-profile=1 --match-with-call-graph 2>&1 | FileCheck %s
 

bolt/test/pie.test

@@ -4,7 +4,7 @@
 ## on Linux systems where the host triple matches the target.
 REQUIRES: system-linux
 
-RUN: %clang %cflags -fPIC -pie %p/Inputs/jump_table_icp.cpp -o %t
+RUN: %clangxx %cxxflags -fPIC -pie %p/Inputs/jump_table_icp.cpp -o %t
 RUN: llvm-bolt %t -o %t.null 2>&1 | FileCheck %s
 
 CHECK: BOLT-INFO: shared object or position-independent executable detected

bolt/test/runtime/X86/instrumentation-indirect.c

@@ -38,7 +38,7 @@ int main(int argc, char **argv) {
 /*
 REQUIRES: system-linux,bolt-runtime,lit-max-individual-test-time
 
-RUN: %clang %cflags %s -o %t.exe -Wl,-q -pie -fpie
+RUN: %clang %cflags -D_GNU_SOURCE %s -o %t.exe -Wl,-q -pie -fpie
 
 RUN: llvm-bolt %t.exe --instrument --instrumentation-file=%t.fdata \
 RUN:   --instrumentation-wait-forks=1 --conservative-instrumentation \

bolt/test/runtime/bolt-reserved.cpp

@@ -5,7 +5,7 @@
  * new sections.
  */
 
-// RUN: %clang %s -o %t.exe -Wl,-q
+// RUN: %clangxx %s -o %t.exe -Wl,-q
 // RUN: llvm-bolt %t.exe -o %t.bolt.exe 2>&1 | FileCheck %s
 // RUN: %t.bolt.exe
 
@@ -16,7 +16,7 @@
  * not enough for allocating new sections.
  */
 
-// RUN: %clang %s -o %t.tiny.exe -Wl,--no-eh-frame-hdr -Wl,-q -DTINY
+// RUN: %clangxx %s -o %t.tiny.exe -Wl,--no-eh-frame-hdr -Wl,-q -DTINY
 // RUN: not llvm-bolt %t.tiny.exe -o %t.tiny.bolt.exe 2>&1 | \
 // RUN:   FileCheck %s --check-prefix=CHECK-TINY