arm64: signal: return struct rt_sigframe from get_sigframe

We only have one type of frame (rt_sigframe) for arm64, so just return
that type directly and dispense with the framesize argument, which is
presumably a hangover from code copied from arch/arm/.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>