replace some raw xmalloc / xrealloc with the XNEW* macros

replace some raw xmalloc / xrealloc with the XNEW* macros

This increases consistancy of how we allocate memory, and always casting the
result to the proper type.  It also helps make sure we get any use of sizeof on
the result type correct.

gas/ChangeLog:

2016-03-22  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* listing.c (listing_message): Use XNEW style allocation macros.
	* read.c (read_a_source_file): Likewise.
	(read_symbol_name): Likewise.
	(s_mri_common): Likewise.
	(assign_symbol): Likewise.
	(s_reloc): Likewise.
	(emit_expr_with_reloc): Likewise.
	(s_incbin): Likewise.
	(s_include): Likewise.
	* sb.c (sb_build): Likewise.
	(sb_check): Likewise.