build: user no-red-zone CRT code

Even though it makes no difference, since the CRT code we pull in from
the toolchain will never touch the red zone in the first place, it is a
good idea to try to be consistent across the build.

cmake/Platforms/x86_64.cmake

-execute_process(COMMAND "x86_64-elf-g++" "-print-file-name=crtbegin.o"
+execute_process(COMMAND "x86_64-elf-g++" "-mno-red-zone" "-print-file-name=crtbegin.o"
   OUTPUT_VARIABLE CRT_BEGIN
   ERROR_QUIET
 )