Add x86 specific compiler flags only when targeting x86

[arunisaac]

I am working on a RISC-V port of wfmash. Several of the CMakeLists.txt files add x86 specific compiler flags. If those flags were added only when wfmash is being built for x86, it would make building for RISC-V easier. Maybe https://cmake.org/cmake/help/latest/variable/CMAKE_SYSTEM_PROCESSOR.html#variable:CMAKE_SYSTEM_PROCESSOR should be checked in CMakeLists.txt before adding x86 specific compiler flags.

Thanks!