gdb.trace/tfind.exp: Force call via global entry point on ppc64le.

gdb.trace/tfind.exp: Force call via global entry point on ppc64le.

tfind.exp sets a breakpoint on *gdb_recursion_test, which is the global
entry point on ppc64le, and won't be hit, since the call uses
the local entry.  Fix by calling the function via a pointer in a global
variable, forcing use of the global entry.

This patch is a slightly modified hunk extracted from
https://sourceware.org/ml/gdb-patches/2015-07/msg00353.html

gdb/testsuite/ChangeLog:

2016-03-09  Wei-cheng Wang  <cole945@gmail.com>
	    Marcin Kościelnicki  <koriakin@0x04.net>

	* gdb.trace/actions.c (gdb_recursion_test_fp): New typedef.
	(gdb_recursion_test_ptr): New global variable.
	(gdb_recursion_test): Call gdb_recursion_test_ptr instead of
	gdb_recursion_test.
	(gdb_c_test): Ditto.