[libc] Remove faccessat entrypoint if faccessat2 syscall is not available (#164936)

[#163091](llvm/llvm-project#163091)

Remove unistd.faccessat entrypoint for x86 linux if faccessat2 syscall
is not available. Tested with non existent symbol and exclusion works.

Author: mleleszi

libc/CMakeLists.txt

@@ -363,7 +363,7 @@ elseif(LLVM_LIBC_FULL_BUILD)
   message(FATAL_ERROR "${LIBC_CONFIG_PATH}/headers.txt file not found and fullbuild requested.")
 endif()
 
-# Check exclude.txt that appends to LIBC_EXCLUDE_ENTRYPOINTS list
+# Check exclude.txt that appends to TARGET_LLVMLIBC_REMOVED_ENTRYPOINTS list
 if(EXISTS "${LIBC_CONFIG_PATH}/exclude.txt")
     include("${LIBC_CONFIG_PATH}/exclude.txt")
 endif()

libc/config/linux/x86_64/exclude.txt

@@ -19,3 +19,11 @@ if(NOT has_sys_random)
     )
   endif()
 endif()
+
+include(CheckSymbolExists)
+check_symbol_exists(SYS_faccessat2 "sys/syscall.h" HAVE_SYS_FACCESSAT2)
+if(NOT HAVE_SYS_FACCESSAT2)
+  list(APPEND TARGET_LLVMLIBC_REMOVED_ENTRYPOINTS
+    libc.src.unistd.faccessat
+  )
+endif()