libgo: use POSIX shell arithmetic expansion

Avoid bash-specific ((expression)) syntax.  As the bash syntax
converts a non-zero value to a zero status (and a zero value to a 1
status), and POSIX arithmetic expansion does not, we have to negate
the result.

Based on patch by Sören Tempel.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/419154

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-ff68b1a147eb60082fd60c198db0ef5477ade938
+a62f20ae78ddd41be682dde8cab075ca4f5dbb2a
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/match.sh

@@ -111,7 +111,7 @@ gobuild() {
     if test "$goarch" != "386"; then
 	line=$(echo "$line" | sed -e "s/\\(${wrap}\\)386\\(${wrap}\\)/\10\2/g")
     fi
-    (($line))
+    return $((!($line)))
 }
 
 matched=

libgo/testsuite/gotest

@@ -302,7 +302,7 @@ gobuild() {
     if test "$goarch" != "386"; then
 	line=$(echo "$line" | sed -e "s/\\(${wrap}\\)386\\(${wrap}\\)/\10\2/g")
     fi
-    (($line))
+    return $((!($line)))
 }
 
 case "x$gofiles" in