Move _obstack_compat out of common

Move _obstack_compat out of common

it is impossible to create an alias of a common symbol (as
compat_symbol does), because common symbols do not have a section or
an offset until linked.  GNU as tolerates aliases of common symbols by
simply creating another common symbol, but other assemblers (notably
LLVM's integrated assembler) are less tolerant.

2013-05-15  Peter Collingbourne  <pcc@google.com>

	* malloc/obstack.c (_obstack_compat): Add initializer.
-