[lld-macho] Require -arch and -platform_version to always be specified

We previously defaulted to x86_64 and an unknown platform, which was fine when
we only supported one arch and did no platform checks, but that will no longer
be true going ahead. Therefore, we should require those flags to be specified
whenever the linker is invoked.

Note that LLD-ELF and ld64 both infer the arch from their input object files,
but the usefulness of that is questionable since clang will always specify these
flags, and most of the time `lld` will be invoked via clang.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D97799