[LLD][MachO] Do not run Mach-O tests on big-endian hosts

Currently, most Mach-O tests fail when executed on a big-endian host. This is
because the Mach-O back-end does not perform the necessary byte swaps when
accessing the (little-endian) binary file format.

For now, simply consider all Mach-O tests unsupported on big-endian hosts,
to enable running the test suite at all on such hosts.

Reviewed by: oontvoo

Differential Revision: https://reviews.llvm.org/D149270