arch: Add some format strings to the parser for reg indexes.

arch: Add some format strings to the parser for reg indexes.

There are two new strings, reg_idx_arr_decl which declares the source
and dest register index arrays, and set_reg_idx_arr which installs them
in the base class.

The set_reg_idx_arr code needs to implicitly figure out what type to use
based on the type of the "this" pointer. The name of the containing
class is not *necessarily* the same as class_name, since the generated
code can use that name, something based on that name, or whatever else
it wants. No other format string (other than class_name itself) uses the
class name internally, so we can't count on that working in existing ISA
definitions.

Change-Id: Id995a46896e71a2fcf3103c34a1e1e67e24f88f4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36878
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>