c++_shared and c++_static are named libc++ and libc++_static in every
other mode. Allow the same names so targets which are built for both
host and device with SDK may specify a single name for the STL.
Test: external/boringssl is able to use "libc++_static" as the STL.
Change-Id: Ibc579c7e24ddb8de12fbfc6e1b5d80648efa589e