Support a "platform-only" version tag.

We tried to support this by version name convention (foo_PLATFORM and
foo_PRIVATE), but not everything follows those conventions. libm has
a LIBC_DEPRECATED, which is a bit to generic to apply this convention
to.

Support a "platform-only" tag which omits the tagged version in the
NDK.

Test: nose2
Bug: None
Change-Id: Iba34628ea02a813d22c8b32d10e54064f17ac6df