For Linux/gnu compatibility, preinclude <stdc-predef.h> if the file is available

As reported in llvm bugzilla 32377.
Here’s a patch to add preinclude of stdc-predef.h.

The gcc documentation says “On GNU/Linux, <stdc-predef.h> is pre-included.” 
See https://gcc.gnu.org/gcc-4.8/porting_to.html;

The preinclude is inhibited with –ffreestanding.

Basically I fixed the failing test cases by adding –ffreestanding which inhibits
this behavior.

I fixed all the failing tests, including some in extra/test, there's a separate
patch for that which is linked here

Patch By: mibintc

Differential Revision: https://reviews.llvm.org/D34158

llvm-svn: 318669