PR jit/63854: Add support for running "make check-jit" under valgrind

PR jit/63854: Add support for running "make check-jit" under valgrind

gcc/testsuite/ChangeLog:
	PR jit/63854
	* jit.dg/jit.exp (report_leak): New.
	(parse_valgrind_logfile): New.
	(fixed_host_execute): Detect if RUN_UNDER_VALGRIND is present
	in the environment, and if so, run the executable under
	valgrind, capturing valgrind's output to a logfile.  Parse the
	log file, generating PASSes and XFAILs for the summary of leaks.
	Use "wait" before "close": valgrind might not have finished
	writing the log out before we parse it, so we need to wait for
	the spawnee to finish.

From-SVN: r218227