GNU C extension introduced in common code

[Robert Lipe]

The patch in:

	http://sourceware.cygnus.com/ml/binutils/2000-01/msg00079.html

makes use of a GNU C extension and therefore breaks compilation on other
compilers.

This might be OK if it's a conscious decision to do so, but there probably
needs to be a better justification.

Since we always link against libiberty, Replacing
		char buff[bfd_coff_filhsz (abfd)];
with
		char *buff = alloca(bfd_coff_filhsz (abfd));
is probably adequate if this really is desirable.

RJL