When recompiling a package whose basename is the name of a standard
package for testing with gccgo, a .o file with the basename of the
package being tested was being placed in the _test/ directory where the
compilation of the test binary then found it when looking for the
standard library package.

This change puts the object files in a separate directory.

Fixes #6793

R=golang-codereviews, dave, gobot, rsc, iant
CC=golang-codereviews
https://golang.org/cl/27650045