gdbstub: Expose gdb_write_register function to consumers of gdbstub

gdbstub: Expose gdb_write_register function to consumers of gdbstub

This patch exposes the gdb_write_register function from
gdbstub/gdbstub.c via the exec/gdbstub.h header file to support use in
plugins to write register contents.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Julian Ganz <neither@nut.email>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Rowan Hart <rowanbhart@gmail.com>
Message-ID: <20250624175351.440780-2-rowanbhart@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250627112512.1880708-6-alex.bennee@linaro.org>