libgo: only add signum to siglist if it doesn't exist yet

This fixes a build issue on musl libc where the same signal number
is used for SIGIO and SIGPOLL.  This causes a compilation error since
the signal numbers must be unique for the signal table.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/400595

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-8336fe4a5da68d9188dfbc4fb647ccbbe4d60ba4
+22b0ccda3aa4d16f770a26a3eb251f8da615c318
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/mksigtab.sh

@@ -26,7 +26,6 @@ SIGLIST=""
 # Handle signals valid on all Unix systems.
 
 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 '^_SIG[A-Z0-9_]*$' >/dev/null 2>&1; then
@@ -34,7 +33,12 @@ addsig() {
         # This is needed for some MIPS signals defined as aliases of other signals
         signum=`grep "const $signum = " gen-sysinfo.go | sed -e 's/.* = //'`
     fi
-    SIGLIST=$SIGLIST"_${signum}_"
+    # Only add signal if the signal number isn't in the list yet.
+    # For example, musl libc uses signal 29 for both SIGIO and SIGPOLL.
+    if ! echo "$SIGLIST" | grep "_${signum}_" >/dev/null 2>&1; then
+        echo "	$1: $2,"
+        SIGLIST=$SIGLIST"_${signum}_"
+    fi
 }
 
 echo '	0:          {0, "SIGNONE: no trap"},'