gcc: configure: drop Valgrind 3.1 compatibility

Our system.h and configure.ac try to accommodate valgrind-3.1, but it is
more than 15 years old at this point. As Valgrind-based checking is a
developer-oriented feature, drop the compatibility stuff and streamline
the detection.

gcc/ChangeLog:

	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Delete manual checks for old Valgrind headers.
	* system.h (VALGRIND_MAKE_MEM_NOACCESS): Delete.
	(VALGRIND_MAKE_MEM_DEFINED): Delete.
	(VALGRIND_MAKE_MEM_UNDEFINED): Delete.
	(VALGRIND_MALLOCLIKE_BLOCK): Delete.
	(VALGRIND_FREELIKE_BLOCK): Delete.