[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>

gdbsupport/common-regcache.h

@@ -96,6 +96,12 @@ struct reg_buffer_common
 				      regcache_register_size (this, regnum)));
   }
 
+  /* Supply part of register REGNUM with zeroed value.  Start at OFFSET in
+     the register, with size SIZE.  The rest of the register is left
+     untouched.  */
+  virtual void raw_supply_part_zeroed (int regnum, int offset, size_t size)
+    = 0;
+
   /* Collect register REGNUM from this register buffer and store its contents in
      DST.  */
   virtual void raw_collect (int regnum, gdb::array_view<gdb_byte> dst) const