[Support] On AIX, Check ENOTSUP on posix_fallocate instead of EOPNOTSUPP

Summary:
`posix_fallocate` can fail if the underlying filesystem does not support
it; and, on AIX, such a failure is reported by a return value of
`ENOTSUP`. The existing code checks only for `EOPNOTSUPP`, which may
share the same value as `ENOTSUP`, but is not required to.

Reviewers: xingxue, sfertile, jasonliu

Reviewed By: xingxue

Subscribers: kristina, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60175

llvm-svn: 357662