[libc] Set the stub filename to the target name instead of the source

The GPU target requires some weird special case handling to create fat
binaries. CMake offers no way to set the name of an object library. The
only way to do this is to create a file with the desired name and use
that. Currently we name it after the source filename. However, this
breaks if there is more than a single source. This patch changes the
logic to instead look up the object target name and use that. E.g.
`src.__support.OSUtil.osutil` will be `osutil.cpp`.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D145912