kselftest/arm64: mte: Use cross-compiler if specified

At the moment we either need to provide CC explicitly, or use a native
machine to get the ARM64 MTE selftest compiled.

It seems useful to use the same (cross-)compiler as we use for the
kernel, so copy the recipe we use in the pauth selftest.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-8-andre.przywara@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>