Do not build the amd64 UEFI loader with GCC

The UEFI loader causes buildworld to fail when building with (in-tree)
GCC, due to a typedef redefinition.  As it happens the in-tree GCC
cannot successfully build the UEFI loader anyhow, as it does not support
__attribute__((ms_abi)).  Thus, just avoid trying to build it with GCC,         rather than disconnecting it from the build until the underlying issue
is fixed.

Sponsored by:	The FreeBSD Foundation