needed for musl 1.2 and time64.
Upstream bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93421

--- gcc-9.2.0/libstdc++-v3/src/c++11/futex.cc.orig	2020-01-20 14:55:05.507548334 -0500
+++ gcc-9.2.0/libstdc++-v3/src/c++11/futex.cc	2020-01-20 14:56:52.458268068 -0500
@@ -61,7 +61,15 @@
 	struct timeval tv;
 	gettimeofday (&tv, NULL);
 	// Convert the absolute timeout value to a relative timeout
+#if defined(SYS_futex_time64)
+	struct
+	  {
+	    long tv_sec;
+	    long tv_nsec;
+	  } rt;
+#else
 	struct timespec rt;
+#endif
 	rt.tv_sec = __s.count() - tv.tv_sec;
 	rt.tv_nsec = __ns.count() - tv.tv_usec * 1000;
 	if (rt.tv_nsec < 0)