gdb: Add tdesc_found_register function to tdesc API

gdb: Add tdesc_found_register function to tdesc API

This commit adds a new function to the target description API within
GDB.  This new function is not used in this commit, but will be used
in the next commit, I'm splitting it out into a separate patch for
easier review.

What I want to do in the next commit is check to see if a target
description supplied a particular register, however, the register in
question could appear in one of two possible features.

The new function allows me to ask the tdesc_arch_data whether a
register was found and assigned a particular GDB register number once
all of the features have been checked.  I think this is a much simpler
solution than adding code such that, while checking each feature, I
spot if the register I'm processing is the one I care about.

No tests here as the new code is not used, but this code will be
exercised in the next commit.