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

cc/gen_stub_libs.py

@@ -69,6 +69,8 @@ def should_omit_version(name, tags, arch, api):
     """
     if version_is_private(name):
         return True
+    if 'platform-only' in tags:
+        return True
     if not symbol_in_arch(tags, arch):
         return True
     if not symbol_in_api(tags, arch, api):