Only build EFI components on supported compilers

As the in-tree GCC does not support attribute((ms_abi)) EFI can only
be built with Clang.

The EFI loader and boot1 validated this, but unused libefi was still built
causing issues under GCC after warnings where enabled by r293724.

Disable building all of EFI when the selected compiler is GCC.

MFC after: 2 weeks
X-MFC-With: r293268
Sponsored by: Multiplay