rseq/selftests: Add support for RISC-V

Add support for RISC-V in the rseq selftests, which covers both
64-bit and 32-bit ISA with little endian mode.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Tested-by: Eric Lin <eric.lin@sifive.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>