Fix breakpoints on symbols with multiple trampoline symbols

On mingw targets it's possible that there are multiple trampoline
symbols for __cxa_throw, in each module where a throw is done, but
without a corresponding global symbol.
Since commit 77f2120b20 they cancel each
other out in search_minsyms_for_name, which makes it impossible to set
a breakpoint there:

(gdb) b __cxa_throw
Function "__cxa_throw" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 2 (__cxa_throw) pending.
(gdb) c
Continuing.
[Inferior 1 (process 10004) exited with code 03]

With catch throw it also doesn't work, and you don't even get an error
message:

(gdb) catch throw
Catchpoint 2 (throw)
(gdb) c
Continuing.
[Inferior 1 (process 5532) exited with code 03]
(gdb)

The fix is to simply ignore other trampoline symbols when looking for
corresponding global symbols, and it's working as expected:

(gdb) b __cxa_throw
Breakpoint 2 at 0x13f091590 (2 locations)
(gdb) c
Continuing.

Breakpoint 2.1, 0x000000013f091590 in __cxa_throw ()
(gdb)

And catch throw also works again:

(gdb) catch throw
Catchpoint 2 (throw)
(gdb) c
Continuing.

Catchpoint 2.1 (exception thrown), 0x000000013f181590 in __cxa_throw ()
(gdb)

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29548
Approved-By: Tom Tromey <tom@tromey.com>

gdb/linespec.c

@@ -4281,6 +4281,10 @@ search_minsyms_for_name (struct collect_info *info,
 	      if (&item2 == &item)
 		continue;
 
+	      /* Ignore other trampoline symbols.  */
+	      if (item2.minsym->type () == mst_solib_trampoline)
+		continue;
+
 	      /* Trampoline symbols can only jump to exported
 		 symbols.  */
 	      if (msymbol_type_is_static (item2.minsym->type ()))