[LibC] Refactor arm64 to aarch64 for darwin. (#141509)

Fixes [#141505](https://github.com/llvm/llvm-project/issues/141505)

The main intention behind this PR is to update the
LibCTargetArchitecture for arm64 to use the implementation of aarch64
architecture and not arm32.

This is a historical issue , and is a blocker to issue #138407. 

The intended fix is to set the `LibCTargetArchitecture` to aarch64 when
it matches arm64 , this in turn would help us run darwin/aarch64
specific code on our MacOs pipeline in the git actions.

Methods used to search and find "darwin/arm" directories was  

1. "find . -type d -path "*/libc/*/darwin/arm" in a linux terminal to
check for arm specifically
2. "find . -type d -path "*/libc/*/darwin/*" to ensure there are no
directories that are named "*arm*" where star is a wildcard for any
character.

libc/cmake/modules/LLVMLibCArchitectures.cmake

@@ -33,12 +33,10 @@ function(get_arch_and_system_from_triple triple arch_var sys_var)
   # value.
   if(target_arch MATCHES "^mips")
     set(target_arch "mips")
-  elseif(target_arch MATCHES "^arm")
-    # TODO(lntue): Shall we separate `arm64`?  It is currently recognized as
-    # `arm` here.
-    set(target_arch "arm")
-  elseif(target_arch MATCHES "^aarch64")
+  elseif(target_arch MATCHES "^aarch64|^arm64")
     set(target_arch "aarch64")
+  elseif(target_arch MATCHES "^arm")
+    set(target_arch "arm")
   elseif(target_arch MATCHES "(x86_64)|(AMD64|amd64)")
     set(target_arch "x86_64")
   elseif(target_arch MATCHES "(^i.86$)")

libc/src/__support/OSUtil/darwin/syscall.h

@@ -15,7 +15,7 @@
 #include "src/__support/macros/properties/architectures.h"
 
 #ifdef LIBC_TARGET_ARCH_IS_ANY_ARM
-#include "arm/syscall.h"
+#include "aarch64/syscall.h"
 #else
 #error "Unsupported architecture"
 #endif