RISC-V documentation cleanups

RISC-V documentation cleanups

A recent mailing list post about install.texi cleanup suggested I take a
look at ours, and there were a few problems:

 * No table of contents entries
 * Not alphabetically ordered
 * Missing a note about requiring binutils-2.28

gcc/ChangeLog:

2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com

        * doc/install.texi (Specific) <riscv32-*-elf>: Add riscv32-*-elf,
        riscv32-*-linux, riscv64-*-elf, riscv64-*-linux to the table of
        contents.
        <riscv64-*-elf>: Re-arrange section
        <riscv32-*-elf>: Add a note about requiring binutils 2.28.
        <riscv32-*-linux>: Likewise.
        <riscv64-*-elf>: Likewise
        <riscv64-*-linux>: Likewise.

From-SVN: r246243