gas: Fix checking for backwards .org with negative offset

gas: Fix checking for backwards .org with negative offset

This patch fixes internal errors in (at least) arm and aarch64 GAS
when assembling code that attempts a negative .org.  The bug appears
to be a regression introduced in binutils-2.29 by commit 9875b36538d.

	* write.c (relax_segment): Fix handling of negative offset when
	relaxing an rs_org frag.
	* testsuite/gas/aarch64/org-neg.d: New test.
	* testsuite/gas/aarch64/org-neg.l: Error output for test.
	* testsuite/gas/aarch64/org-neg.s: Input for test.
	* testsuite/gas/arm/org-neg.d: New test.
	* testsuite/gas/arm/org-neg.l: Error output for test.
	* testsuite/gas/arm/org-neg.s: Input for test.