MesaLib: enable llvm option on all x86 platforms, and not on non-x86.

It's for the AMDGPU driver. I don't know of any ARM boards, but that may
change in the future. highly unlikely for SPARC.

Also lift the minimum GCC requirement for linux llvm option - we use GCC_REQD
for the llvm package, and gcc48 works at least on CentOS 6.8 builds, so it'll
build fine.
Linux is will benefit from the AMDGPU driver, although it requires a new
kernel anyway.

bump PKGREVISION.

(maya)