arm64/insn: introduce aarch64_insn_gen_{nop|branch_imm}() helper functions

arm64/insn: introduce aarch64_insn_gen_{nop|branch_imm}() helper functions

This is copied from Linux 4.7, and the initial commit
that put this in is 5c5bf25d4f7a950382f94fc120a5818197b48fe9
"arm64: introduce aarch64_insn_gen_{nop|branch_imm}() helper functions"

This lays the groundwork for Livepatch to generate the
trampoline to jump to the new replacement function.
Also allows us to NOP the callsites.

Acked-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
--
Cc: Ross Lagerwall <ross.lagerwall@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>
RFC: First submission
v1: The full copy of insn_gen_branch instead of just the code to make branch
v2: Added Julien's Ack.
    Remove the duplicate paragraph in the commit message.