Backport KWIML commit 6fc81d8816 (abi.h: Update RISC-V support for
revised macro names, 2017-12-30) to the CMake 3.10 release branch.
The upstream RISC-V project changed some compiler symbols so `__riscv__`
must now be renamed `__riscv`.
Patch-by: Richard W.M. Jones <rjones@redhat.com>