cmd/go/internal/tool: set Internal.ExeName on tool's package
While the cached name of an executable is set based on the base name of
the package path, the executable produced as the output of link doesn't
have ExeName set on it and is just called a.out (with a .exe suffix on
Windows). Set ExeName so that the first time the binary is run, from the
directory link is run in, it has the right name for ps.