BACKPORT: arm64: alternative: Merge alternative-asm.h into alternative.h

asm/alternative-asm.h and asm/alternative.h are extremely similar,
and really deserve to live in the same file (as this makes further
modufications a bit easier).

Fold the content of alternative-asm.h into alternative.h, and
update the few users.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Bug: 31432001
Change-Id: I2d5e2f619bba5439453bd26bf4a20d9449a79322
(cherry picked from commit 8d883b23aed73cad844ba48051c7e96eddf0f51c)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>