gas: allow frag address wrapping in absolute section

This:
	 .struct -1
	x:
	 .fill 1
	y:
results in an internal error in frag_new due to abs_section_offset
wrapping from -1 to 0.  Frags in the absolute section don't do much so
I think we can allow the address wrap.

	* frags.c (frag_new): Allow address wrap in absolute section.