Better test harness

[mattico]

I'd like to have results like:

target: thumbv6m-none-eabi

Testing 123 intrinsics...
test muldi ... ok
test mulosi ... ok
test mulodi ... ok
test divdi3 ... ERROR
  input: (-1, INT_MAX)
  results:
    compiler-rt:       32768
    gcc_s:             32768
    compiler-builtins: 32767
test udivmoddi4 ... ERROR
  input: (25, 367)
  results:
    compiler-rt:       3
    gcc_s:             N/A
    compiler-builtins: 365
...

target: thumbv6m-none-eabi

Benchmarking 123 intrinsics...
bench muldi...
  compiler-rt:       16,386 ns/iter (+- 366)
  gcc_s:             14,375 ns/iter (+- 540)
  compiler-builtins: 17,690 ns/iter (+- 750)
bench mulosi...
...