Hack libc++ support header to work in the platform.

Upstream now supports using ToT libc++ with old NDKs, but as such it
is now *only* compatible with the NDK. That will need to be fixed both
for the platorm and for the NDK-in-platform use case since neither has
android/ndk-version.h.

Test: m checkbuild
Bug: http://b/119587551
Change-Id: Ib8a2c16e608925f4786f81538fdb41b4814ef5f7