arm64: memory: Implement __tag_set() as common function

There's no need for __tag_set() to be a complicated macro when
CONFIG_KASAN_SW_TAGS=y and a simple static inline otherwise. Rewrite
the thing as a common static inline function.

Tested-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>