Namespace the reg class to avoid clashes with OS headers

Namespace the reg class to avoid clashes with OS headers

Fix build issues on NetBSD where the reg symbol exists in public headers.

regformats/regdef.h:22:8: error: redefinition struct
 struct reg
        ^~~
/usr/include/amd64/reg.h:51:8: note: previous definition struct
 struct reg {
        ^~~

gdb/ChangeLog:

	* regformats/regdef.h: Put reg in gdb namespace.

gdbserver/ChangeLog:

	* regcache.cc (find_register_by_number): Update.
	* tdesc.cc (init_target_desc): Likewise.
	* tdesc.h (target_desc::reg_defs): Likewise.