arm64: Move patching utilities out of instruction encoding/decoding

arm64: Move patching utilities out of instruction encoding/decoding

Files insn.[c|h] containt some functions used for instruction patching.
In order to reuse the instruction encoder/decoder, move the patching
utilities to their own file.

Signed-off-by: Julien Thierry <jthierry@redhat.com>
Link: https://lore.kernel.org/r/20210303170536.1838032-2-jthierry@redhat.com
[will: Include patching.h in insn.h to fix header mess; add __ASSEMBLY__ guards]
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>