[clang+lld] Pass -platform_version args to ld64.lld
Fix regression where we aren't passing -platform_version to new ld64.lld after D95204: [lld-macho] Switch default to new Darwin backend.
Most of the changes were originally in D95204, but I backed them out due to
test failures on builds which have CLANG_DEFAULT_LINKER=lld. The tests are
properly updated in this diff.
Reviewed By: #lld-macho, thakis
Differential Revision: https://reviews.llvm.org/D97741