[libc++] Refactor max_size.pass.cpp

Reorganize the test and simplify the #ifdefs. Fix a typo in __powerpc64__
as a fly-by, and also add a test for the unstable ABI.

Differential Revision: https://reviews.llvm.org/D124403

NOKEYCHECK=True
GitOrigin-RevId: 0e2fb8ac84bd033450a828409671390a5b6430ac
1 file changed