RISC-V: Document the medlow and medany code models

RISC-V: Document the medlow and medany code models

This documentation is patterned off the aarch64 -mcmodel documentation.

gcc/ChangeLog:

2017-11-01  Palmer Dabbelt  <palmer@dabbelt.com>

        * doc/invoke.texi (RISC-V Options): Explicitly name the medlow
        and medany code models, and describe what they do.

From-SVN: r254321