[Driver] Fix compiler-rt lookup for x32

x86_64-linux-gnu and x86_64-linux-gnux32 use different ABIs and objects
built for one cannot be used for the other. In order to build and use
compiler-rt for x32, we need to treat x32 as a new arch there. This
updates the driver to search using the new arch name.

Reviewed By: glaubitz

Differential Revision: https://reviews.llvm.org/D100148