kselftest/arm64: Convert tpidr2 test to use kselftest.h

Recent work by Thomas Weißschuh means that it is now possible to use
kselftest.h with nolibc. Convert the tpidr2 test which is nolibc specific
to use kselftest.h, making it look more standard and ensuring it gets the
benefit of any work done on kselftest.h.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250609-kselftest-arm64-nolibc-header-v1-1-16ee1c6fbfed@kernel.org


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