only use _darwin10_Unwind_FindEnclosingFunction on darwin >= 10

only use _darwin10_Unwind_FindEnclosingFunction on darwin >= 10

gcc:
	* unwind-dw2-fde-darwin.c (_darwin10_Unwind_FindEnclosingFunction):
	Dunmmy function with NULL return unless the target is
	OSX >= 10.6 (Darwin10).
libjava:
	* include/posix.h: Make substitution of
	_darwin10_Unwind_FindEnclosingFunction conditional on
	OSX >= 10.6 (Darwin10).

From-SVN: r163309