gdb/testsuite/gdb.base/stap-probe: Minor clean-up

gdb/testsuite/gdb.base/stap-probe: Minor clean-up

This patch resolves a couple of issues with the test case for SystemTap
user-space probe points:
  1. The preprocessor macro guarding the semaphore variables in the C
     file is (rather confusingly) named USE_PROBES. This has been
     renamed to USE_SEMAPHORES, to better reflect its function.
  2. The test procedures in the expect file improperly pass the flag
     defining USE_PROBES to prepare_for_testing; as such, the test
     binary that's supposed to have probes with semaphores is the same
     as the one without. This has also been fixed.
  3. No test is performed to check that `info probes' returns
     information about probe semaphores. Such a test is included in this
     patch.

gdb/testsuite/ChangeLog
2020-01-10  George Barrett  <bob@bob131.so>

	* gdb.base/stap-probe.c: Rename USE_PROBES to USE_SEMAPHORES.
	* gdb.base/stap-probe.exp: Likewise.
	(stap_test): Pass argument as an additional flag.
	(stap_test_no_debuginfo): Likewise.
	(stap_test): Check `info probes stap' output for semaphore
	addresses if the test binary is supposed to have them.