Prefer int-wide pg_atomic_flag over char-wide when using gcc intrinsics.

configure can only probe the existence of gcc intrinsics, not how well
they're implemented, and unfortunately the answer is sometimes "badly".
In particular we've found that multiple compilers fail to implement
char-width __sync_lock_test_and_set() correctly on PPC; and even a correct
implementation would necessarily be pretty inefficient, since that hardware
has only a word-wide primitive to work with.

Given the knowledge we've accumulated in s_lock.h, it appears that it's
best to rely on int-width TAS operations on most non-Intel architectures.
Hence, pick int not char when both are nominally available to us in
generic-gcc.h (note that that code is not used for x86[_64]).

Back-patch to fix regression test failures on FreeBSD/PPC.  Ordinarily
back-patching a change like this would be verboten because of ABI breakage.
But since pg_atomic_flag is not yet used in any Postgres data structure,
there's no ABI to break.  It seems safer to back-patch to avoid possible
gotchas, if someday we do back-patch something that uses pg_atomic_flag.

Discussion: https://postgr.es/m/25414.1483076673@sss.pgh.pa.us