libstdc++: Fix several check-simd interaction issues

libstdc++: Fix several check-simd interaction issues

libstdc++-v3/ChangeLog:

	* testsuite/experimental/simd/driver.sh (verify_test): Print
	test output on run xfail. Do not repeat lines from the log that
	were already printed on stdout.
	(test_selector): Make the compiler flags pattern usable as a
	substring selector.
	(toplevel): Trap on SIGINT and remove the log and sum files.
	Call timout with --foreground to quickly terminate on SIGINT.
	* testsuite/experimental/simd/generate_makefile.sh: Simplify run
	targets via target patterns. Default DRIVEROPTS to -v for run
	targets. Remove log and sum files after completion of the run
	target (so that it's always recompiled).
	Place help text into text file for reasonable 'make help'
	performance.