[Bug target/100266] [RISCV] Provide programmatic implementation of CAS

[christophm30 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100266

--- Comment #1 from Christoph M. <christophm30 at gmail dot com> ---
A patchset to resolve this can be found here:
  https://gcc.gnu.org/pipermail/gcc-patches/2021-April/568684.html