[AIX][z/OS][Support] Provide alternate no-op mapped_file_region::dontNeedImpl implementation

mapped_file_region::dontNeedImpl added in D116366 calls madvise, which
causes problems for z/OS and AIX.

For z/OS, we don't have either madvise, so treat this as a no-op, same
as Windows does.

For AIX, it doesn't have any effect, doesn't have a standardized
signature, and it needs certain feature test macros (i.e. _ALL_SOURCE)
we don't set by default for LLVM on AIX, so just make it a no-op too.

Differential Revision: https://reviews.llvm.org/D116603