Make gdb.base/jit.exp binaries unique

Make gdb.base/jit.exp binaries unique

This testcase compiles the same program and library differently
multiple times using the same file names.  Make them unique, to make
it easier to debug test problems.

gdb/testsuite/ChangeLog:
2016-03-31  Pedro Alves  <palves@redhat.com>

	PR gdb/19858
	* gdb.base/jit.exp (compile_jit_test): Add intro comment.  Add
	BINSUFFIX parameter, and handle it.
	(top level): Adjust calls compile_jit_test.