gdb: use put_frame_register instead of put_frame_register_bytes in pseudo_to_concat_raw

Here, we write single complete registers, we don't need the
functionality of put_frame_register_bytes, use put_frame_register
instead.

Change-Id: I987867a27249db4f792a694b47ecb21c44f64d08
Approved-By: Tom Tromey <tom@tromey.com>