Merge commit 47b0262d3 from llvm git (by me):

Add <stdarg.h> include to kmp_os.h, to get the va_list type, required
after cde8f4c164a2. Sort system includes, while here.

The original merged commit works fine by itself on head, but fails to
compile on stable branches because stdarg.h is not implicitly pulled in.

MFC after: immediately, to fix failing builds on stable/{11,12}