Add a "fork" of musl as //fusl.

fusl/src/thread/pthread_sigmask.c

Index: fusl/src/thread/pthread_sigmask.c
diff --git a/fusl/src/thread/pthread_sigmask.c b/fusl/src/thread/pthread_sigmask.c
new file mode 100644
index 0000000000000000000000000000000000000000..88c333f6b682b0c3db71fc35b5e7ed69636272a4
--- /dev/null
+++ b/fusl/src/thread/pthread_sigmask.c
@@ -0,0 +1,19 @@
+#include <signal.h>
+#include <errno.h>
+#include "syscall.h"
+
+int pthread_sigmask(int how, const sigset_t *restrict set, sigset_t *restrict old)
+{
+	int ret;
+	if ((unsigned)how - SIG_BLOCK > 2U) return EINVAL;
+	ret = -__syscall(SYS_rt_sigprocmask, how, set, old, _NSIG/8);
+	if (!ret && old) {
+		if (sizeof old->__bits[0] == 8) {
+			old->__bits[0] &= ~0x380000000ULL;
+		} else {
+			old->__bits[0] &= ~0x80000000UL;
+			old->__bits[1] &= ~0x3UL;
+		}
+	}
+	return ret;
+}