benchmarks/libcelero: fix build on GCC architectures and powerpc64 elfv2

clang 8 doesn't have full support for powerpc, powerpc64 or powerpcspe, so use
GCC there.
On powerpc64 elfv2, the default is to build for elfv1, so pass a flag to fix it.

MFH: 2020Q2 (fix build blanket)