[Driver] Fix incorrect GNU triplet for PowerPC on SUSE Linux

Summary:
On SUSE distributions for 32-bit PowerPC, gcc is configured
as a 64-bit compiler using the GNU triplet "powerpc64-suse-linux",
but invoked with "-m32" by default. Thus, the correct GNU triplet
for 32-bit PowerPC SUSE distributions is "powerpc64-suse-linux"
and not "powerpc-suse-linux".

Reviewers: jrtc27, nemanjai, glaubitz

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D55326