runtime: add special handling for signal 34

The musl libc uses signal 34 internally for setgid (similar to how glibc
uses signal 32 and signal 33).  For this reason, special handling is
needed for this signal in the runtime. The gc implementation already
handles the signal accordingly.  As such, this commit intends to
simply copy the behavior of the Google Go implementation to libgo.

See https://go.dev/issues/39343

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/400594

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-62fc90f52da2f52cbe3b4f10e560dc6aa59baeb5
+8336fe4a5da68d9188dfbc4fb647ccbbe4d60ba4
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/signal_gccgo.go

@@ -106,7 +106,8 @@ func getsig(i uint32) uintptr {
 	if sigaction(i, nil, &sa) < 0 {
 		// On GNU/Linux glibc rejects attempts to call
 		// sigaction with signal 32 (SIGCANCEL) or 33 (SIGSETXID).
-		if GOOS == "linux" && (i == 32 || i == 33) {
+		// On musl signal 34 (SIGSYNCCALL) also needs to be treated accordingly.
+		if GOOS == "linux" && (i == 32 || i == 33 || i == 34) {
 			return _SIG_DFL
 		}
 		throw("sigaction read failure")

libgo/mksigtab.sh

@@ -95,10 +95,12 @@ checksig _SIGLOST '   {_SigNotify, "SIGLOST: resource lost (Sun); server died (G
 
 # Special handling of signals 32 and 33 on GNU/Linux systems,
 # because they are special to glibc.
+# Signal 34 is additionally special to Linux systems with musl.
 if test "${GOOS}" = "linux"; then
-    SIGLIST=$SIGLIST"_32__33_"
+    SIGLIST=$SIGLIST"_32__33__34_"
     echo '	32: {_SigSetStack + _SigUnblock, "signal 32"}, /* SIGCANCEL; see issue 6997 */'
     echo '	33: {_SigSetStack + _SigUnblock, "signal 33"}, /* SIGSETXID; see issues 3871, 9400, 12498 */'
+    echo '	34: {_SigSetStack + _SigUnblock, "signal 34"}, /* musl SIGSYNCCALL; see issue 39343 */'
 fi
 
 if test "${GOOS}" = "aix"; then