system.h (gcc_assert): Invoke __builtin_unreachable() instead of fancy_abort() if !ENABLE_ASSERT_CHECKING.

	* system.h (gcc_assert): Invoke __builtin_unreachable() instead of
	fancy_abort() if !ENABLE_ASSERT_CHECKING.
	(gcc_unreachable): Invoke __builtin_unreachable() if
	!ENABLE_ASSERT_CHECKING.

From-SVN: r150091