[release/2.1] Fix Process.ExitCode on mac for killed processes (#29407)

[joperezr]

This is a port of #29407 into the release/2.1 branch that will fix a regression on Process.ExitCode on mac for processes that are killed.

There was one small merge conflict on

https://github.com/joperezr/corefx/blob/806c5b270e299a8b4a06310c192b4e612f9e8607/src/Native/Unix/System.Native/pal_process.h#L215-L225

that required me to get rid of the DLLEXPORT keywords at the beginning of those two functions. Nothing else had to be changed.

cc: @tmds @stephentoub @danmosemsft @wtgodbe

Fixes: #29370

Master PR commit message:

• Fix Process.ExitCode on mac for killed processes
  Use SIGCHLD to trigger Process waitpid check #26291 changed process reaping
  from using waitpid to waitid. This caused a regression on mac, since
  for processes that are killed, (on mac) waitid does not return the
  signal number that caused the process to terminated.
  We change back to waitpid for reaping children and determining the
  exit code. waitid is used to find terminated children.
  Fixes https://github.com/dotnet/corefx/issues/29370
• TestExitCodeKilledChild: remove runtime check
• TestExitCodeKilledChild: remove greater than assert

[wtgodbe]

Thanks Jose! Did you also send email to shiproom?

[joperezr]

@wtgodbe doing that now 😄

[tmds]

LGTM, Thanks!