[clang] Allow using -rtlib=platform to switching to the default rtlib on all targets

Normally, passing -rtlib=platform overrides any earlier -rtlib
options, and overrides any hardcoded CLANG_DEFAULT_RTLIB option.
However, some targets, MSVC and Darwin, have custom logic for
disallowing specific -rtlib= option values; amend these checks for
allowing the -rtlib=platform option.

Differential Revision: https://reviews.llvm.org/D132444