[compiler-rt] Build libfuzzer sources with the chosen C++ compiler

I was getting build failures due to missing <cstddef> when building the
libfuzzer tests. It turns out that the custom command was using
COMPILER_RT_TEST_COMPILER when building the source file rather than
the COMPILER_RT_TEST_CXX_COMPILER.

Pull Request: #83090

Author: arichardson

compiler-rt/cmake/Modules/CompilerRTCompile.cmake

@@ -70,9 +70,14 @@ function(clang_compile object_file source)
   if (TARGET CompilerRTUnitTestCheckCxx)
     list(APPEND SOURCE_DEPS CompilerRTUnitTestCheckCxx)
   endif()
+  string(REGEX MATCH "[.](cc|cpp)$" is_cxx ${source_rpath})
+  if (is_cxx)
+    set(compiler ${COMPILER_RT_TEST_COMPILER})
+  else()
+    set(compiler ${COMPILER_RT_TEST_CXX_COMPILER})
+  endif()
   if(COMPILER_RT_STANDALONE_BUILD)
     # Only add global flags in standalone build.
-    string(REGEX MATCH "[.](cc|cpp)$" is_cxx ${source_rpath})
     if(is_cxx)
       string(REPLACE " " ";" global_flags "${CMAKE_CXX_FLAGS}")
     else()
@@ -102,7 +107,7 @@ function(clang_compile object_file source)
 
   add_custom_command(
     OUTPUT ${object_file}
-    COMMAND ${COMPILER_RT_TEST_COMPILER} ${compile_flags} -c
+    COMMAND ${compiler} ${compile_flags} -c
             -o "${object_file}"
             ${source_rpath}
     MAIN_DEPENDENCY ${source}