arm/alternatives: Rename alt_instr fields which are used in common code

arm/alternatives: Rename alt_instr fields which are used in common code

Alternatives auditing for livepatches is currently broken.  To fix it, the
livepatch code needs to inspect more fields of alt_instr.

Rename ARM's fields to match x86's, because:

 * ARM already exposes alt_offset under the repl name via ALT_REPL_PTR().
 * "alt" is ambiguous in a structure entirely about alternatives already.
 * "repl", being the same width as orig leads to slightly neater code.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit 418cf59c4e29451010d7efb3835b900690d19866)