gas: ensure XCOFF DWARF subsection are initialized to 0

debug_abbrev doesn't use end_exp to compute its size. However, it must
be NULL. Otherwise, ppc_xcoff_end might try to access uninitialized
memory.

gas/
	* config/tc-ppc.c (ppc_dwsect): Use XCNEW instead of XNEW when creating
	a new subsection.