gdb: convert target debug macros to functions

Convert all the macros to static functions.  Some macros were unused,
and now that they are functions, got flagged by the compiler, so I
omitted them.

No behavior change expected.

Change-Id: Ia88e61d95e29a0378901c71aa50df7c37d16bebe
Approved-By: Tom Tromey <tom@tromey.com>