gdb: rename gdbarch_pseudo_register_write to gdbarch_deprecated_pseudo_register_write

The next patch introduces a new variant of gdbarch_pseudo_register_write
that takes a frame instead of a regcache for implementations to write
raw registers.  Rename to old one to make it clear it's deprecated.

Change-Id: If8872c89c6f8a1edfcab983eb064248fd5ff3115
Reviewed-By: John Baldwin <jhb@FreeBSD.org>