Define missing F_RDLCK, F_WRLCK and F_UNLCK constants on linux-s390x
Define missing F_RDLCK, F_WRLCK and F_UNLCK constants on linux-s390x

These are defined in the fcntl.h glibc header on s390x systems
on Linux but missing in the libc crate, so add them as they are
required for the file locking API in rustc.

rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang/libc