/usr/include/x86_64-linux-musl/bits/ is in musl-dev 0.9.15-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/x86_64-linux-musl/bits/alltypes.h
/usr/include/x86_64-linux-musl/bits/endian.h
/usr/include/x86_64-linux-musl/bits/errno.h
/usr/include/x86_64-linux-musl/bits/fcntl.h
/usr/include/x86_64-linux-musl/bits/fenv.h
/usr/include/x86_64-linux-musl/bits/float.h
/usr/include/x86_64-linux-musl/bits/io.h
/usr/include/x86_64-linux-musl/bits/ioctl.h
/usr/include/x86_64-linux-musl/bits/ipc.h
/usr/include/x86_64-linux-musl/bits/limits.h
/usr/include/x86_64-linux-musl/bits/mman.h
/usr/include/x86_64-linux-musl/bits/msg.h
/usr/include/x86_64-linux-musl/bits/posix.h
/usr/include/x86_64-linux-musl/bits/reg.h
/usr/include/x86_64-linux-musl/bits/setjmp.h
/usr/include/x86_64-linux-musl/bits/shm.h
/usr/include/x86_64-linux-musl/bits/signal.h
/usr/include/x86_64-linux-musl/bits/socket.h
/usr/include/x86_64-linux-musl/bits/stat.h
/usr/include/x86_64-linux-musl/bits/statfs.h
/usr/include/x86_64-linux-musl/bits/stdarg.h
/usr/include/x86_64-linux-musl/bits/stdint.h
/usr/include/x86_64-linux-musl/bits/syscall.h
/usr/include/x86_64-linux-musl/bits/termios.h
/usr/include/x86_64-linux-musl/bits/user.h