jit: supply MULTILIB_DEFAULTS as arguments when invoking driver

jit: supply MULTILIB_DEFAULTS as arguments when invoking driver

gcc/jit/ChangeLog:
	* jit-playback.c (invoke_driver): Convert local "argvec"
	to an auto_argvec, so that it owns copies of the strings,
	rather than borrows them, updating ADD_ARG to use xstrdup
	and special-casing the NULL terminator to avoid
	xstrdup (NULL).  Call add_multilib_driver_arguments at the front
	of the arguments.
	(MULTILIB_DEFAULTS): Provide a default definition.
	(multilib_defaults_raw): New constant array.
	(gcc::jit::playback::context::add_multilib_driver_arguments): New
	method.
	* jit-playback.h
	(gcc::jit::playback::context::add_multilib_driver_arguments): New
	method.
	* docs/internals/test-hello-world.exe.log.txt: Update.
	* docs/_build/texinfo/libgccjit.texi: Regenerate.

From-SVN: r226126