RISC-V: Set SLOW_BYTE_ACCESS=1

RISC-V: Set SLOW_BYTE_ACCESS=1

When implementing the RISC-V port, I took the name of this macro at
face value.  It appears we were mistaken in what this means, here's a
quote from the SPARC port that better describes what SLOW_BYTE_ACCESS
does

    /* Nonzero if access to memory by bytes is slow and undesirable.
       For RISC chips, it means that access to memory by bytes is no
       better than access by words when possible, so grab a whole word
       and maybe make use of that.  */

I've added the comment to our port as well.

See https://gcc.gnu.org/ml/gcc/2017-08/msg00202.html for more
discussion.  Thanks to Michael Clark and Andrew Pinski for the help!

gcc/ChangeLog

2017-11-04  Andrew Waterman  <andrew@sifive.com>

        * config/riscv/riscv.h (SLOW_BYTE_ACCESS): Change to 1.

From-SVN: r254416