Use gcc_fallthrough() instead of __attribute__((fallthrough)

Use gcc_fallthrough() instead of __attribute__((fallthrough)

Without this my build breaks on older compilers like GCC-5.4.

gcc/ChangeLog:

2017-03-13  Kito Cheng  <kito.cheng@gmail.com>

       * config/riscv/riscv.c (riscv_emit_float_compare>: Use
       gcc_fallthrough() instead of __attribute__((fallthrough));

From-SVN: r246116