ehdr_start twiddles

ehdr_start twiddles

We force __ehdr_start to defined in before_allocation, then restore
the type and a union after dynamic symbols have been allocated.
The union contains a number of structs, all starting with a "next"
pointer, and various fields depending on the symbol type.  The old
code restored the entire union, but it isn't necessary to restore the
"next" pointer, and in fact it could be wrong if __ehdr_start happened
to be last on the list and some symbols were added before restoring.

	* emultempl/elf32.em (before_allocation): Don't restore
	__ehdr_start u.*.next pointer.