libgo: don't use grep -q in mksigtab.sh

Solaris grep does not support the -q option.
    
    Reviewed-on: https://go-review.googlesource.com/79239

From-SVN: r255042

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-fecb92bda0aa6d70c89d14635ff568df77d2bb5f
+178c00f929e6268f24f0c9dd9eb69d3d14d8096e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/mksigtab.sh

@@ -29,7 +29,7 @@ addsig() {
     echo "	$1: $2,"
     # Get the signal number and add it to SIGLIST
     signum=`grep "const $1 = " gen-sysinfo.go | sed -e 's/.* = //'`
-    if echo "$signum" | grep -q '^_SIG[A-Z0-9_]*$'; then
+    if echo "$signum" | grep '^_SIG[A-Z0-9_]*$' >/dev/null 2>&1; then
         # Recurse once to obtain signal number
         # This is needed for some MIPS signals defined as aliases of other signals
         signum=`grep "const $signum = " gen-sysinfo.go | sed -e 's/.* = //'`