gdbstub: Replace GET_REG*() macros with gdb_get_reg*() functions

gdbstub: Replace GET_REG*() macros with gdb_get_reg*() functions

This avoids polluting the global namespace with a non-prefixed macro and
makes it obvious in the call sites that we return.

Semi-automatic conversion using, e.g.,
 sed -i 's/GET_REGL(/return gdb_get_regl(mem_buf, /g' target-*/gdbstub.c
followed by manual tweaking for sparc's GET_REGA() and Coding Style.

Acked-by: Michael Walle <michael@walle.cc> (for lm32)
Acked-by: Max Filippov <jcmvbkbc@gmail.com> (for xtensa)
Signed-off-by: Andreas Färber <afaerber@suse.de>