gas: always add dummy symbols when creating XCOFF sections.

gas: always add dummy symbols when creating XCOFF sections.

Most of the algorithms for XCOFF in tc-ppc.c assume that
the csects field of a ppc_xcoff_section isn't NULL.
This was already made for most of the sections with the creation
of a dummy symbol.
This patch simply mades it default when creating a xcoff_section.

gas/
	* config/tc-ppc.c (ppc_init_xcoff_section): Always create
	the dummy symbol.
	(md_begin): Adjust ppc_init_xcoff_section call.
	(ppc_comm): Likewise.
	(ppc_change_csect): Likewise.