UPSTREAM: arm64: alternative: Provide if/else/endif assembler macros

The existing alternative_insn macro has some limitations that make it
hard to work with. In particular the fact it takes instructions from it
own macro arguments means it doesn't play very nicely with C pre-processor
macros because the macro arguments look like a string to the C
pre-processor. Workarounds are (probably) possible but things start to
look ugly.

Introduce an alternative set of macros that allows instructions to be
presented to the assembler as normal and switch everything over to the
new macros.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Bug: 31432001
Change-Id: I8564b766fc4e54afbcde0707bcd784a195594fbd
(cherry picked from commit 63e40815f02584ba8174e0f6af40924b2b335cae)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>