arm64: klib: bitops: fix unpredictable stxr usage

We're currently relying on unpredictable behaviour in our testops
(test_and_*_bit), as stxr is unpredictable when the status register and
the source register are the same

This patch changes reallocates the status register so as to bring us back into
the realm of predictable behaviour. Boot tested on an AEMv8 model.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>