[llvm-rc]: Find <target>-clang over just clang

This patch makes llvm-rc/windres prefer <target>-clang over
clang when doing it's preprocessing. This is so that we can
have a .cfg file for <target> and configure sysroot and other
important flags.

Config files not picked up with clang --target=<target>
automatically.

We only look for <target>-clang in the same dir as llvm-windres
and not for all PATHs to minimize the change.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D119219