RISC-V: Add satp as an alias for sptbr

RISC-V: Add satp as an alias for sptbr

The RISC-V privileged ISA changed the name of sptbr (Supervisor Page
Table Base Register) to satp (Supervisor Address Translation and
Protection) to reflect the fact it could be used for more than just
paging.  This patch adds an alias, as they're the same register.

include/ChangeLog

2017-11-06  Palmer Dabbelt  <palmer@dabbelt.com>

        * opcode/riscv-opc.h (sptbr): Rename to satp.
        (CSR_SPTBR): Rename to CSR_SATP.
        (sptbr): Alias to CSR_SATP.

gas/ChangeLog

2017-11-06  Palmer Dabbelt  <palmer@dabbelt.com>

        * testsuite/gas/riscv/satp.d: New test.
        testsuite/gas/riscv/satp.s: Likewise.
        testsuite/gas/riscv/riscv.exp: Likewise.
        config/tc-riscv.c (md_begin): Handle CSR aliases.