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

fusl/src/mman/mmap.c

+#include <unistd.h>
+#include <sys/mman.h>
+#include <errno.h>
+#include <stdint.h>
+#include <limits.h>
+#include "syscall.h"
+#include "libc.h"
+
+static void dummy(void) { }
+weak_alias(dummy, __vm_wait);
+
+#define UNIT SYSCALL_MMAP2_UNIT
+#define OFF_MASK ((-0x2000ULL << (8*sizeof(long)-1)) | (UNIT-1))
+
+void *__mmap(void *start, size_t len, int prot, int flags, int fd, off_t off)
+{
+        if (off & OFF_MASK) {
+                errno = EINVAL;
+                return MAP_FAILED;
+        }
+        if (len >= PTRDIFF_MAX) {
+                errno = ENOMEM;
+                return MAP_FAILED;
+        }
+        if (flags & MAP_FIXED) {
+                __vm_wait();
+        }
+#ifdef SYS_mmap2
+        return (void *)syscall(SYS_mmap2, start, len, prot, flags, fd, off/UNIT);
+#else
+        return (void *)syscall(SYS_mmap, start, len, prot, flags, fd, off);
+#endif
+}
+
+weak_alias(__mmap, mmap);
+
+LFS64(mmap);