LLVM hardcoded to build for X86

[branfosj]

For example https://github.com/easybuilders/easybuild-easyconfigs/blob/develop/easybuild/easyconfigs/l/LLVM/LLVM-10.0.0-GCCcore-8.3.0.eb#L35

-DLLVM_TARGETS_TO_BUILD=X86

I discovered this while building numba on POWER9, but the above is in most LLVM EasyConfigs. We can either detect architecture and set it to X86 or PowerPC as required; or we can set it to all.