Reland 2nd attempt: [lld-macho] Fix bug in reading cpuSubType field.

This reverts commit 52a118d0.

New changes:
Fix tests to dump both slices in the fat-archive because otool
isn't deterministic about which slice it prints across different archs.
(It printed x86 on x86 machines but arm64 on arm64, this was why
the test failed on arm64)

Differential Revision: https://reviews.llvm.org/D139572