runtime: handle signal 34 for musl setgid

It has been observed that setgid hangs when using cgo with musl.
This fix ensures that signal 34 gets handled in an appropriate way,
like signal 33 when using glibc.

Fixes #39343

Change-Id: I89565663e2c361f62cbccfe80aaedf290bd58d57
Reviewed-on: https://go-review.googlesource.com/c/go/+/236518
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: GeorgeTsilias

src/runtime/sigtab_linux_generic.go

@@ -45,7 +45,7 @@ var sigtable = [...]sigTabT{
 	/* 31 */ {_SigThrow, "SIGSYS: bad system call"},
 	/* 32 */ {_SigSetStack + _SigUnblock, "signal 32"}, /* SIGCANCEL; see issue 6997 */
 	/* 33 */ {_SigSetStack + _SigUnblock, "signal 33"}, /* SIGSETXID; see issues 3871, 9400, 12498 */
-	/* 34 */ {_SigNotify, "signal 34"},
+	/* 34 */ {_SigSetStack + _SigUnblock, "signal 34"}, /* musl SIGSYNCCALL; see issue 39343 */
 	/* 35 */ {_SigNotify, "signal 35"},
 	/* 36 */ {_SigNotify, "signal 36"},
 	/* 37 */ {_SigNotify, "signal 37"},

src/runtime/sigtab_linux_mipsx.go

@@ -42,7 +42,7 @@ var sigtable = [...]sigTabT{
 	/*  31 */ {_SigNotify, "SIGXFSZ: file size limit exceeded"},
 	/*  32 */ {_SigSetStack + _SigUnblock, "signal 32"}, /* SIGCANCEL; see issue 6997 */
 	/*  33 */ {_SigSetStack + _SigUnblock, "signal 33"}, /* SIGSETXID; see issues 3871, 9400, 12498 */
-	/*  34 */ {_SigNotify, "signal 34"},
+	/*  34 */ {_SigSetStack + _SigUnblock, "signal 34"}, /* musl SIGSYNCCALL; see issue 39343 */
 	/*  35 */ {_SigNotify, "signal 35"},
 	/*  36 */ {_SigNotify, "signal 36"},
 	/*  37 */ {_SigNotify, "signal 37"},