Fix Fortran STOP.

Fix Fortran STOP.

The minimal libgfortran setup was created for NVPTX, but will also be used by
AMD GCN.

This patch simply removes an assumption that NVPTX is the only user.
Specifically, NVPTX exit is broken, but AMD GCN exit works just fine.

2018-09-12  Andrew Stubbs  <ams@codesourcery.com>

	libgfortran/
	* runtime/minimal.c (exit): Only work around nvptx bugs on nvptx.

From-SVN: r264238