dev-libs/beignet: fix multilib libdir detection for LLVM-4+

dev-libs/beignet: fix multilib libdir detection for LLVM-4+

By default Beignet CMake scripts look for LLVM parameters using
llvm-config regardless of target architecture. For multilib builds on
amd64 against >=sys-devel/llvm-4.0.0 this resulted in the linker being
passed the abi_x86_64 libdir in abi_x86_32 mode, causing "file in wrong
format" errors and build failures.

We now work around this issue by having Beignet's FindLLVM.cmake look
for ${CHOST}-llvm-config instead. Hopefully there will be an official
upstream fix for this at some point.

Closes: https://bugs.gentoo.org/638078
Package-Manager: Portage-2.3.13, Repoman-2.3.3