cmd/go: test: do not put object files where later steps will find them
cmd/go: test: do not put object files where later steps will find them

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 issue 6793

R=golang-codereviews, dave, gobot, rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/27650045

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang