runtime: skip darwin osinit_hack on ios

Darwin needs the osinit_hack call to fix some bugs in the Apple libc
that surface when Go programs call exec. On iOS, the functions that
osinit_hack uses are not available, so signing fails. But on iOS exec
is also unavailable, so the hack is not needed. Disable it there,
which makes signing work again.

Fixes golang#58323.

Change-Id: I3f1472f852bb36c06854fe1f14aa27ad450c5945
(cherry picked from commit e95f1f1cedaa8284bb00dec23da292ef3821cd2e)

src/runtime/sys_darwin.go

@@ -213,7 +213,9 @@ func pthread_kill_trampoline()
 //
 //go:nosplit
 func osinit_hack() {
-	libcCall(unsafe.Pointer(abi.FuncPCABI0(osinit_hack_trampoline)), nil)
+	if GOOS == "darwin" { // not ios
+		libcCall(unsafe.Pointer(abi.FuncPCABI0(osinit_hack_trampoline)), nil)
+	}
 	return
 }
 func osinit_hack_trampoline()