gdb: add value::allocate_register

Add value::allocate_register, to facilitate allocating a value
representing a register in a given frame (or rather, in the given
frame's previous frame).  It will be used in a subsequent patch.  I
changed one relatively obvious spot that could use it, to at least
exercise the code path.

Change-Id: Icd4960f5e471a74b657bb3596c88d89679ef3772
Reviewed-By: John Baldwin <jhb@FreeBSD.org>