PerfTest::assemble functions return results.

PerfTest::assemble functions return results.

gdb/testsuite/ChangeLog:

	* lib/perftest.exp (PerfTest::compile): Unconditionally call body.
	(PerfTest::startup): New function.
	(PerfTest::run): Return result of calling body.
	(PerfTest::assemble): Rewrite.
	* gdb.perf/backtrace.exp (PerfTest::assemble): Update function result.
	* gdb.perf/disassemble.exp (PerfTest::assemble): Ditto.
	* gdb.perf/single-step.exp (PerfTest::assemble): Ditto.
	* gdb.perf/skip-prologue.exp (PerfTest::assemble): Ditto.
	* gdb.perf/solib.exp (PerfTest::assemble): Ditto.