[gdb/tdep] Use raw_supply_part_zeroed for AArch64

In gdb/aarch64-linux-tdep.c we find:
...
      gdb::byte_vector za_zeroed (za_bytes, 0);
      regcache->raw_supply (tdep->sme_za_regnum, za_zeroed);
...

We can't use reg_buffer::raw_supply_zeroed here because only part of the
register is written.

Add raw_supply_part_zeroed, and use it instead.

Likewise elsewhere in AArch64 tdep code.

Tested on aarch64-linux.

Approved-By: Luis Machado <luis.machado@arm.com>