RISC-V: Build Infastructure

RISC-V: Build Infastructure

This patch lays out the top-level orginazition of the RISC-V port.  It
contains all the Implies files as well as various other fragments of
build infastructure for the RISC-V port.  This contains the only change
to a shared file: config.h.in.

RISC-V is a family of base ISAs with optional extensions.  The base ISAs
are RV32I and RV64I, which are 32-bit and 64-bit integer-only ISAs, but
this port currently only supports RV64I based systems.  Support for
RISC-V lives in in sysdeps/riscv.  In addition to these ISAs, our glibc
port supports most of the currently-defined extensions: the A extension
for atomics, the M extension for multiplication, the C extension for
compressed instructions, and the F/D extensions for single/double
precision IEEE floating-point.  Most of these extensions are handled by
GCC, but glibc defines various floating-point wrappers and emulation
routines as well as some atomic wrappers.

We support running glibc-based programs on Linux, the support for which
lives in sysdeps/unix/sysv/linux/riscv.

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

        * sysdeps/riscv/Implies: New file.
        * sysdeps/riscv/Makefile: Likewise.
        * sysdeps/riscv/configure: Likewise.
        * sysdeps/riscv/configure.ac: Likewise.
        * sysdeps/riscv/nptl/Makefile: Likewise.
        * sysdeps/riscv/preconfigure: Likewise.
        * sysdeps/riscv/rv64/Implies-after: Likewise.
        * sysdeps/riscv/rv64/rvd/Implies: Likewise.
        * sysdeps/riscv/rv64/rvf/Implies: Likewise.
        * sysdeps/unix/sysv/linux/riscv/Implies: Likewise.
        * sysdeps/unix/sysv/linux/riscv/Makefile: Likewise.
        * sysdeps/unix/sysv/linux/riscv/Versions: Likewise.
        * sysdeps/unix/sysv/linux/riscv/configure: Likewise.
        * sysdeps/unix/sysv/linux/riscv/configure.ac: Likewise.
        * sysdeps/unix/sysv/linux/riscv/ldd-rewrite.sed: Likewise.
        * sysdeps/unix/sysv/linux/riscv/rv64/Implies: Likewise.
        * sysdeps/unix/sysv/linux/riscv/rv64/Makefile: Likewise.
        * sysdeps/unix/sysv/linux/riscv/shlib-versions: Likewise.