gdb: fix refactoring hiccup in rs6000_register_to_value

gdb: fix refactoring hiccup in rs6000_register_to_value

In 2023-12-14 "gdb: make get_frame_register_bytes take the next frame"
(9fc79b42369), *_register_to_value functions were made to (a) call
get_next_frame_sentinel_okay (frame) (b) pass that next frame to
get_frame_register_bytes.

Step (b) was omitted for rs6000-tdep.c; this manifests as a regression on
PPC platforms for e.g. O2_float_param: instead of seeing…

  Temporary breakpoint 1, callee.increment (val=val@entry=99.0, msg=...) at callee.adb:19

… we get "optimized_out" for val.  Passing next_frame to
get_frame_register_bytes fixes the issue.

Approved-By: Simon Marchi <simon.marchi@efficios.com>