Make gdb.PendingFrame.read_register handle "user" registers.

Make gdb.PendingFrame.read_register handle "user" registers.

The C function, pending_framepy_read_register(), which implements
the python interface gdb.PendingFrame.read_register does not handle
the so called "user" registers like "pc".  An assertion error is
triggered due to the user registers having numbers larger than or
equal to gdbarch_num_regs(gdbarch).

With the VALUE_FRAME_ID tweak in place, the call to
get_frame_register_value() can simply be replaced by a call to
value_of_register(), which handles both real registers as well as the
user registers.

gdb/ChangeLog:

	* python/py-unwind.c (pending_framepy_read_register): Use
	value_of_register() instead of get_frame_register_value().