UPSTREAM: arm64: add alternative runtime patching

With a blatant copy of some x86 bits we introduce the alternative
runtime patching "framework" to arm64.
This is quite basic for now and we only provide the functions we need
at this time.
This is connected to the newly introduced feature bits.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Bug: 31432001
Change-Id: I2420b40a45a2206b05415fdcdf72ef210cba25a3
(cherry picked from commit e039ee4ee3fcf174736f2cb0a2eed6cb908348a6)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>