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.

Change-Id: I4c64fa15de3994a1a5c7c5ec0de6a82ddd858917
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/419154
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

libgo/match.sh

diff --git a/libgo/match.sh b/libgo/match.sh
index 7ed587f..e5ed98d 100755
--- a/libgo/match.sh
+++ b/libgo/match.sh
@@ -111,7 +111,7 @@
     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

diff --git a/libgo/testsuite/gotest b/libgo/testsuite/gotest
index 04e4267..0a0a7e1 100755
--- a/libgo/testsuite/gotest
+++ b/libgo/testsuite/gotest
@@ -302,7 +302,7 @@
     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