util: don't try to emit half-float intrinsics if avx isn't available

util: don't try to emit half-float intrinsics if avx isn't available

These instructions only have vex encodings, thus they can't be used without
avx. (Technically, one can still use avx-128 if avx isn't available because
the environment doesn't store the ymm registers, however I don't think llvm
can.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>