Makefile.inc1: Allow showconfig and test-system-* if (X)CC is GCC

This case gets hit in make universe on Linux, since we will first run
make test-system-compiler to determine whether to use the system or
universe toolchain, during which time CC is the host's, GCC, and XCC
isn't set, so defaults to the same.

Fixes: 4c0dfd5959bd ("arm: fail early on gcc builds")