gdb: pass more const target_waitstatus by reference

gdb: pass more const target_waitstatus by reference

While working on target_waitstatus changes, I noticed a few places where
const target_waitstatus objects could be passed by reference instead of
by pointers.  And in some cases, places where a target_waitstatus could
be passed as const, but was not.  Convert them as much as possible.

Change-Id: Ied552d464be5d5b87489913b95f9720a5ad50c5a