syscall: build export_unix_test.go on solaris
syscall: build export_unix_test.go on solaris

Patch from Rainer Orth.

Change-Id: I688147ef57d1e88cb7d10038efc54b5790019fb0
Reviewed-on: https://go-review.googlesource.com/29436
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend