frag_alloc use of obstack_alloc

frag_alloc use of obstack_alloc

Avoid the alignment hackery necessary when obstack_alloc is used.
obstack_alloc expands to obstack_blank plus obstack_finish, and the
latter call is where alignment of the tail of the obstack happens.

The docs say obstack_alloc "is invoked almost like malloc", which
implies a fixed size allocation and you don't need other obstack calls
in its use.  So I think trying to use obstack_alloc in frag_alloc was
always a poor choice.

	* frags.c (frag_alloc): Replace obstack_alloc with obstack_blank.