Remove cleanups from check_fast_tracepoint_sals

Remove cleanups from check_fast_tracepoint_sals

This changes the gdbarch fast_tracepoint_valid_at method to use a
std::string as its out parameter, and then updates all the uses.  This
allows removing a cleanup from breakpoint.c.

Regression tested by the buildbot.

ChangeLog
2018-02-24  Tom Tromey  <tom@tromey.com>

	* i386-tdep.c (i386_fast_tracepoint_valid_at): "msg" now a
	std::string.
	* gdbarch.sh (fast_tracepoint_valid_at): Change "msg" to a
	std::string*.
	* gdbarch.c: Rebuild.
	* gdbarch.h: Rebuild.
	* breakpoint.c (check_fast_tracepoint_sals): Use std::string.
	* arch-utils.h (default_fast_tracepoint_valid_at): Update.
	* arch-utils.c (default_fast_tracepoint_valid_at): "msg" now a
	std::string*.