Avoid accidentally using the host's native 'as' command.

When invoking clang for the host to assemble .S files, the -B flag
would not be provided, which allowed the host prebuilt clang to
use an 'as' from the native environment. Most of the time this
"just works", but some newer 'as' versions cause problems with
the older prebuilt toolchain, for example by generating
unsupported relocation types.

To avoid this problem, simply use the -B flag to tell clang to
invoke the assembler from the correct prebuilt gcc prefix.

Change-Id: I560ba91ecca723b19637f3d96269c8ddd0c79e00