Fix a link error when compiled without optimization option "-O2" (#844)

Error log:
  libriscv.a(processor.o): In function `state_t::reset(processor_t*, unsigned long)':
  .../_build/../riscv/processor.cc:509: undefined reference to `state_t::num_triggers'
  collect2: error: ld returned 1 exit status
  Makefile:349: recipe for target 'spike-log-parser' failed

Signed-off-by: Eric Tang <tangxingxin1008@gmail.com>