kexec: arm64/ia64: Increase the scope of kexec_flush_icache_page()

ia64 does icache maintenance for each crash kernel page that will
be used in-place. It has a hook in kimage_load_crash_segment() to
do this.
arm64 similarly needs to do 'PoC' maintenance for these pages, but
does this when protecting the crashkernel region by unmapping it.

As the distinction between crashkernel and regular kexec is about
to be blurred, arm64 needs to do this maintenance for in-place
pages.

Re-use the kexec_flush_icache_page() hook, renaming it
arch_kexec_flush_page() to cover any cache maintenance the arch
code needs. Switch this to a weak symbol instead of a pre-processor
macro so the header include order can't silently break this.

Signed-off-by: James Morse <james.morse@arm.com>
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I've only build-tested the ia64 changes.