[clang+lld] Pass -platform_version args to ld64.lld

Fix regression where we aren't passing `-platform_version` to new ld64.lld after {D95204}.

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