[driver] Ensure --help=params lines end with period

[driver] Ensure --help=params lines end with period

Ensure that gcc --help=params lines end with a period by:
- fixing the help message of param HOT_BB_COUNT_FRACTION, and
- adding a test-case.

Build and tested on x86_64.

2018-11-21  Tom de Vries  <tdevries@suse.de>

	PR driver/79855
	* params.def (HOT_BB_COUNT_FRACTION): Terminate help message with
	period.

	* lib/options.exp (check_for_options_with_filter): New proc.
	* gcc.misc-tests/help.exp: Check that --help=params lines end with
	period.

From-SVN: r266338