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

fusl/src/thread/pthread_sigmask.c

+#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;
+}