tools include: Add some common function attributes

We don't have definitions of __always_unused or __noreturn in the tools
version of compiler.h, add them so we can use them in kselftests.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230728-arm64-signal-memcpy-fix-v4-3-0c1290db5d46@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>