[clang] stop baremetal driver to append .a to lib

When the clang baremetal driver selects the rt.builtins static library
it prefix with "-l" and appends ".a". The result is a nonsense option
which lld refuses to accept.

Differential Revision: https://reviews.llvm.org/D73904

Change-Id: Ic753b6104e259fbbdc059b68fccd9b933092d828