re PR go/81617 (mksigtab.sh fails to resolve NSIG with glibc 2.26)

PR go/81617
    libgo: change mksigtab to recognize glibc 2.26 NSIG expression
    
    Fixes golang/go#21147
    Fixes GCC PR 81617
    
    Reviewed-on: https://go-review.googlesource.com/52611

From-SVN: r250858

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c1ac6bc99f988633c6bc68a5ca9ffad3487750ef
+adac632f95d1cd3421c9c1df5204db10b6a92c44
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/mksigtab.sh

@@ -107,6 +107,19 @@ if test "${GOOS}" = "aix"; then
     nsig=`expr $nsig + 1`
 else
     nsig=`grep 'const _*NSIG = [0-9]*$' gen-sysinfo.go | sed -e 's/.* = \([0-9]*\)/\1/'`
+    if test -z "$nsig"; then
+	if grep 'const _*NSIG = [ (]*_*SIGRTMAX + 1[ )]*' gen-sysinfo.go >/dev/null 2>&1; then
+	    rtmax=`grep 'const _*SIGRTMAX = [0-9]*$' gen-sysinfo.go | sed -e 's/.* = \([0-9]*\)/\1/'`
+	    if test -n "$rtmax"; then
+		nsig=`expr $rtmax + 1`
+	    fi
+	fi
+    fi
+fi
+
+if test -z "$nsig"; then
+    echo 1>&2 "could not determine number of signals"
+    exit 1
 fi
 
 i=1