RISC-V: Linux Syscall Interface

Contains the Linux system call interface, as well as the definitions of
a handful of system calls.

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

        * sysdeps/riscv/nptl/nptl-sysdep.S: New file.
        * sysdeps/unix/sysv/linux/riscv/arch-fork.h: Likewise.
        * sysdeps/unix/sysv/linux/riscv/clone.S: Likewise.
        * sysdeps/unix/sysv/linux/riscv/profil-counter.h: Likewise.
        * sysdeps/unix/sysv/linux/riscv/pt-vfork.S: Likewise.
        * sysdeps/unix/sysv/linux/riscv/syscall.c: Likewise.
        * sysdeps/unix/sysv/linux/riscv/sysdep.S: Likewise.
        * sysdeps/unix/sysv/linux/riscv/sysdep.h: Likewise.
        * sysdeps/unix/sysv/linux/riscv/vfork.S: Likewise.