[mach-o] add support for round tripping all arm/thumb relocations

Update the parse-arm-relocs.yaml test case to run the linker back to back
to ensure all relocations round trip in and out of mach-o.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@213700 91177308-0d34-0410-b5e6-96231b3b80d8