Explicitly define __STDC_FORMAT_MACROS for PRIu64

Summary:
Builds are failing on RHEL machines because of PRIu64.

lvm/projects/compiler-rt/lib/gwp_asan/guarded_pool_allocator.cpp:420:50: error: expected ')'

`snprintf(ThreadBuffer, kThreadBufferLen, "%" PRIu64, ThreadID);`
inttypes.h in RHEL uses PRIu64 macros only when __STDC_FORMAT_MACROS is defined.

Author: DTharun

Reviewers: hctim

Reviewed By: hctim

Differential Revision: https://reviews.llvm.org/D64388

llvm-svn: 365801