gdb: add all_breakpoints function

gdb: add all_breakpoints function

Introduce the all_breakpoints function, which returns a range that can
be used to iterate on breakpoints.  Replace all uses of the
ALL_BREAKPOINTS macro with this.

In one instance, I could replace the breakpoint iteration with a call to
get_breakpoint.

gdb/ChangeLog:

	* breakpoint.c (ALL_BREAKPOINTS): Remove, replace all uses with
	all_breakpoints.
	(breakpoint_iterator): New.
	(breakpoint_range): New.
	(all_breakpoints): New.

Change-Id: I229595bddad7c9100b179a9dd56b04b8c206e86c