compiler, runtime: Add explicit checks for zero and overflow division.

	* lang.opt: Add -fgo-check-divide-zero and
	-fgo-check-divide-overflow.
	* gccgo.texi (Invoking gccgo): Document new options.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/branches/gcc-4_7-branch@186638 138bc75d-0d04-0410-961f-82ee72b054a4