Let stddef.h redefine NULL if __need_NULL is set, as needed by glibc, PR12997.

See the bug and the cfe-commits thread "[patch] Let stddef.h redefine NULL if
__need_NULL is set" for discussion.

Fixes PR12997 and is similar to the __need_wint_t bits already in this file.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207482 91177308-0d34-0410-b5e6-96231b3b80d8