kselftest/arm64: mte: Output warning about failing compiler

kselftest/arm64: mte: Output warning about failing compiler

At the moment we check the compiler's ability to compile MTE enabled
code, but guard all the Makefile rules by it. As a consequence a broken
or not capable compiler just doesn't do anything, and make happily
returns without any error message, but with no programs created.

Since the MTE feature is only supported by recent aarch64 compilers (not
all stable distro compilers support it), having an explicit message
seems like a good idea. To not break building multiple targets, we let
make proceed without errors.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-9-andre.przywara@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>