selftests/nolibc: always compile the kernel with GCC

selftests/nolibc: always compile the kernel with GCC

LLVM/clang can not build the kernel for all architectures supported by
nolibc. The current setup uses the same compiler to build the kernel as
is used for nolibc-test. This prevents using the full qemu-system tests
for LLVM builds.

Instead always build the kernel with GCC. For the nolibc testsuite the
kernel does not need to be built with LLVM.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250719-nolibc-llvm-system-v1-3-1730216ce171@weissschuh.net