riscv.h: Add a temporary workaround for #582
riscv.h: Add a temporary workaround for #582

The workaround can be enabled by passing -DISSUE_582_WORKAROUND to the
compiler.

Signed-off-by: David Lattimore <ca2b3b3a0cb4dbefeffc88a538fc9a81415614de@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground