Fix regression on aarch64-linux gdbserver

Commit 9a03f218 ("Fix gdb.base/watchpoint-unaligned.exp on aarch64")
fixed a watchpoint bug in gdb -- but did not touch the corresponding
code in gdbserver.

This patch moves the gdb code into gdb/nat, so that it can be shared
with gdbserver, and then changes gdbserver to use it, fixing the bug.

This is yet another case where having a single back end would prevent
bugs.

I tested this using the AdaCore internal gdb testsuite.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29423


Approved-By: Luis Machado <luis.machado@arm.com>