RISC-V: Atomic and Locking Routines

This patch implements various atomic and locking routines on RISC-V.  We
mandate the A extension on Linux-capable RISC-V systems, so this can
rely on always having the various atomic instructions availiable.

2018-01-29  Palmer Dabbelt  <palmer@sifive.com>

        * sysdeps/riscv/nptl/bits/pthreadtypes-arch.h: New file.
        * sysdeps/riscv/nptl/bits/semaphore.h: Likewise.
        * sysdeps/riscv/nptl/libc-lowlevellock.c: Likewise.
        * sysdeps/unix/sysv/linux/riscv/atomic-machine.h: Likewise.