FROMLIST: arm64/lib: copy_page: avoid x18 register in assembler code

Register x18 will no longer be used as a caller save register in the
future, so stop using it in the copy_page() code.

Bug: 145210207
Change-Id: I521c1e52899b36f6a53269852b7b1b2858280756
(am from https://lore.kernel.org/patchwork/patch/1149064/)
Link: https://patchwork.kernel.org/patch/9836869/


Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
[Sami: changed the offset and bias to be explicit]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>