make obstack object allocators more type-safe

make obstack object allocators more type-safe

This changes OBSTACK_ZALLOC and OBSTACK_CALLOC to cast their value to
the correct type.  This is more type-safe and also is more in line
with the other object-allocation macros in libiberty.h.

Making this change revealed one trivial error in dwarf2read.c.
On the whole that seems pretty good to me.

Tested by rebuilding.

2014-06-20  Tom Tromey  <tromey@redhat.com>

	* dwarf2read.c (dw2_get_real_path): Use correct type in
	OBSTACK_CALLOC.
	* gdb_obstack.h (OBSTACK_ZALLOC, OBSTACK_CALLOC): Cast result.