Use thr_stksegment to determine Solaris stack base

	* os_dep.c [SOLARIS_STACKBOTTOM] (GC_solaris_stack_base): New
	function.
	[!BEOS && !AMIGA && !MSWIN32 && !MSWINCE && !OS2 && !NOSYS &&
	!ECOS] (GC_get_stack_base): Use it.
	* include/private/gcconfig.h [SPARC && SUNOS5]
	(SOLARIS_STACKBOTTOM): Define.
	(STACKBOTTOM, HEURISTIC2): Remove.
	[I386 && SUNOS5]
	(SOLARIS_STACKBOTTOM): Define.
	(STACKBOTTOM): Remove.

From-SVN: r196490