testmain.exp: link against GOLIBS

Patch by Maciej W. Rozycki.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/207458

From-SVN: r278316

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-2d0504236c7236345ee17a0cb43a3bb9ce3acf7f
+25d5e9dca49ad3f49393b254dd87f8df51487c65
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/testsuite/libgo.testmain/testmain.exp

@@ -47,7 +47,11 @@ if [info exists gluefile] {
     regsub $gluefile $object_files "" object_files
 }
 
-set comp_output [go_target_compile "$object_files _testmain.go" \
+set golibs ""
+if [info exists env(GOLIBS)] {
+    set golibs "$env(GOLIBS)"
+}
+set comp_output [go_target_compile "$object_files _testmain.go $golibs" \
 			 "./a.exe" "executable" $options]
 if ![ string match "" $comp_output ] {
     verbose -log $comp_output