Turn global 'ggc_force_collect' variable into 'force_collect' parameter to 'ggc_collect'

This simplifies the interface and gets us rid of a global variable.
No change in behavior.

Clean-up for 2004-09-02 CVS commit (Subversion r86974,
Git commit 07724022)
"Better memory statistics, take 2".

	gcc/
	* ggc.h (ggc_collect): Add 'force_collect' parameter.
	* ggc-page.c (ggc_collect): Use that one instead of global
	'ggc_force_collect'.  Adjust all users.
	* doc/gty.texi (Invoking the garbage collector): Update.
	* ggc-internal.h (ggc_force_collect): Remove.
	* ggc-common.c (ggc_force_collect): Likewise.
	* selftest.h (forcibly_ggc_collect): Remove.
	* ggc-tests.c (selftest::forcibly_ggc_collect): Likewise.
	* read-rtl-function.c (test_loading_labels): Adjust.
	* selftest-run-tests.c (run_tests): Likewise.