[X86] Remove the mm_malloc.h include guard hack from the X86 builtins tests

The X86 clang/test/CodeGen/*builtins.c tests define the mm_malloc.h include
guard as a hack for avoiding its inclusion (mm_malloc.h requires a hosted
environment since it expects stdlib.h to be available - which is not the case
in these internal clang codegen tests).
This patch removes this hack and instead passes -ffreestanding to clang cc1.

Differential Revision: https://reviews.llvm.org/D24825

llvm-svn: 282581