Allow stdatomic.h to be included from mingw prebuilt.

Define __GNUC_PREREQ if it isn't already defined.
This is a bit ugly, but it seems to be easiest to make this header as
context-independent as possible.  Together with the addition of a symbolic
link to this file from the mingw prebuilt, this appears to unblock
commit e9fa6be333e35d9e7ae435aeb32532875b95fe22.

Change-Id: I97e39cda8d8f9aa108aa61c4121da09eb9739062