kselftest/arm64: Add a test case for TPIDR2 restore

Due to the fact that TPIDR2 is intended to be managed by libc we don't
currently test modifying it via the signal context since that might
disrupt libc's usage of it and cause instability. We can however test the
opposite case with less risk, modifying TPIDR2 in a signal handler and
making sure that the original value is restored after returning from the
signal handler. Add a test which does this.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230621-arm64-fix-tpidr2-signal-restore-v2-2-c8e8fcc10302@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>