C entrypoint fixes

[zachcmadsen]

A couple fixes related to microsoft/one-collect#268:

• Recognize <module>!_start as a valid top frame
• Bump symbol length by one for dynamic symbols to catch addresses on the symbol end boundary. _start seems to be a pathological case

With the one-collect change and these fixes we get a complete stack:

+ Process32 workload (140228) Args: workload
 + Thread (140228) CPU=19984ms
  + workload!_start
  |+ libc.so.6!__libc_start_main
  | + libc.so.6!__libc_start_call_main
  |  + workload!main
  |   + workload!run_workload
  |    + workload!hot_loop
  |    + workload!warm_loop
  |    + workload!cold_loop

[zachcmadsen]

Cc @brianrob

[brianrob]

LGTM. Thanks @zachcmadsen!