runtime: add the --without-libatomic configure option

This adds the --without-libatomic configure option, which is useful for building libgo
with a non-gcc compiler.

It disables libgo's dependency on libatomic. This
is useful for platforms where it is known that the libatomic runtime
functions are not required, or where the compiler automatically
provides an implementation of them.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@211065 138bc75d-0d04-0410-961f-82ee72b054a4