* include/lightning.h, include/lightning/jit_private.h,
	lib/jit_names.c, lib/lightning.c: Add initial support
	for the new jit_va_start, jit_va_arg, jit_va_arg_d, and
	jit_va_end interfaces. The jit_va_start call is supposed
	to return a va_list compatible pointer, but not yet
	decided if it will be "declared" stdarg compatible,
	as for now only x86 support has been added (and should
	be compatible), but issues may arise on other backends.

	* check/lightning.c: Add wrappers to call the new jit_va_*
	interfaces.

	* lib/jit_x86-cpu.c, lib/jit_x86.c: Implement the new
	jit_va_* for x86.

	* lib/jit_x86-sz.c: Add fields, but not yet fully updated,
	as this is an intermediate commit.

	* lib/jit_aarch64-sz.c, lib/jit_aarch64.c,
	lib/jit_alpha-sz.c, lib/jit_alpha.c,
	lib/jit_arm-sz.c, lib/jit_arm.c,
	lib/jit_hppa-sz.c, lib/jit_hppa.c,
	lib/jit_ia64-sz.c, lib/jit_ia64.c,
	lib/jit_mips-sz.c, lib/jit_mips.c,
	lib/jit_ppc-sz.c, lib/jit_ppc.c,
	lib/jit_s390-sz.c, lib/jit_s390.c,
	lib/jit_sparc-sz.c, lib/jit_sparc.c: Prepare for the
	new jit_va_* interfaces. Not yet implemented, and will
	cause an assertion if used.

	* check/va_list.tst: Simple early test case, that works
	on x86_64, x32, ix86, cygwin, and cygwin64.