Add interrupt test cases using clint MSW and MTIMER interrupts

requires 
riscv-software-src/riscv-config#169, 
riscv-software-src/riscof#106

To include these tests in riscof testlist flow, add Smclint to riscof yaml file, e.g.:
spike/spike_isa.yaml:
  ISA: RV32IMCZicsr_Zifencei_Smclint



Signed-off-by: Dan Smathers <dan.smathers@seagate.com>

Author: dansmathers