Introduce -finstrument-functions-once

The goal is to make it possible to use it in (large) production binaries
to do function-level coverage, so the overhead must be minimum and, in
particular, there is no protection against data races so the "once"
moniker is imprecise.

gcc/
	* common.opt (finstrument-functions): Set explicit value.
	(-finstrument-functions-once): New option.
	* doc/invoke.texi (Program Instrumentation Options): Document it.
	* gimplify.cc (build_instrumentation_call): New static function.
	(gimplify_function_tree): Call it to emit the instrumentation calls
	if -finstrument-functions[-once] is specified.
gcc/testsuite/
	* gcc.dg/instrument-4.c: New test.