cmd/compile: create test binary in temp directory

cmd/compile: create test binary in temp directory

The new TestDashS was leaving a dreg "test" file in
cmd/compile/internal/gc.  Create it in the temporary directory instead.

Also change path.Join to filepath.Join throughout global_test.go.

Change-Id: Ib7707fada2b3ab5e8abc2ba74e4c402821c1408b
Reviewed-on: https://go-review.googlesource.com/19965
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>