[libc] Support basic 'complex.h' functionality on GPUs (#189999)

Summary:
This PR adds the basic complex functions needed by some targets. This is
an attempt to provide the standard interface needed by
'https://github.com/llvm/llvm-project/pull/187831'

libc/config/gpu/amdgpu/entrypoints.txt

@@ -279,6 +279,20 @@ set(TARGET_LIBC_ENTRYPOINTS
 )
 
 set(TARGET_LIBM_ENTRYPOINTS
+    # complex.h entrypoints
+    libc.src.complex.creal
+    libc.src.complex.crealf
+    libc.src.complex.creall
+    libc.src.complex.cimag
+    libc.src.complex.cimagf
+    libc.src.complex.cimagl
+    libc.src.complex.conj
+    libc.src.complex.conjf
+    libc.src.complex.conjl
+    libc.src.complex.cproj
+    libc.src.complex.cprojf
+    libc.src.complex.cprojl
+
     # math.h entrypoints
     libc.src.math.acos
     libc.src.math.acosf

libc/config/gpu/amdgpu/headers.txt

@@ -1,6 +1,7 @@
 set(TARGET_PUBLIC_HEADERS
     libc.include.assert
     libc.include.ctype
+    libc.include.complex
     libc.include.string
     libc.include.strings
     libc.include.signal

libc/config/gpu/nvptx/entrypoints.txt

@@ -279,6 +279,20 @@ set(TARGET_LIBC_ENTRYPOINTS
 )
 
 set(TARGET_LIBM_ENTRYPOINTS
+    # complex.h entrypoints
+    libc.src.complex.creal
+    libc.src.complex.crealf
+    libc.src.complex.creall
+    libc.src.complex.cimag
+    libc.src.complex.cimagf
+    libc.src.complex.cimagl
+    libc.src.complex.conj
+    libc.src.complex.conjf
+    libc.src.complex.conjl
+    libc.src.complex.cproj
+    libc.src.complex.cprojf
+    libc.src.complex.cprojl
+
     # math.h entrypoints
     libc.src.math.acos
     libc.src.math.acosf

libc/config/gpu/nvptx/headers.txt

@@ -1,6 +1,7 @@
 set(TARGET_PUBLIC_HEADERS
     libc.include.assert
     libc.include.ctype
+    libc.include.complex
     libc.include.string
     libc.include.strings
     libc.include.signal