gdb: convert nat/x86-dregs.c macros to functions

I'm debugging why GDB crashes on OpenBSD/amd64, turns out it's because
x86_dr_low.get_status is nullptr.  It would have been useful to be able
to break on x86_dr_low_get_status, so I thought it would be a good
reason to convert these function-like macros into functions.

Change-Id: Ic200b50ef8455b4697bc518da0fa2bb704cf4721
1 file changed