Non-parallel "make test-spec" caused
spec/ruby/core/process/wait2_spec.rb failures because mspec
uses "exec" in single-process mode, so there's no chance
the post-exec state could know about the MJIT child process
from its pre-exec state.

[ruby-core:87846] [Bug #14867]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63877 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

This is not working with cppflags="-DMJIT_FORCE_ENABLE" on my machine.

```
$ make test-spec
$ /home/k0kubun/src/github.com/ruby/ruby-svn/.ruby-force/miniruby -I/home/k0kubun/src/github.com/ruby/ruby-svn/lib /home/k0kubun/src/github.com/ruby/ruby-svn/tool/runruby.rb --archdir=/home/k0kubun/src/github.com/ruby/ruby-svn/.ruby-force --extout=.ext -- /home/k0kubun/src/github.com/ruby/ruby-svn/spec/mspec/bin/mspec-run -B ../spec/default.mspec
ruby 2.6.0dev (2018-07-07 trunk 63874) +JIT [x86_64-linux]
[| | ================  40%                    | 00:02:03]      0F      0E leaked before wait2 specs: [[31406, #<Process::Status: pid 31406 exit 0>]]

1)
An exception occurred during: before :all FAILED
Expected [[31406, #<Process::Status: pid 31406 exit 0>]] to be empty
/home/k0kubun/src/github.com/ruby/ruby-svn/spec/ruby/core/process/wait2_spec.rb:18:in `block (3 levels) in <top (required)>'
/home/k0kubun/src/github.com/ruby/ruby-svn/spec/ruby/core/process/wait2_spec.rb:16:in `block (2 levels) in <top (required)>'
/home/k0kubun/src/github.com/ruby/ruby-svn/spec/ruby/core/process/wait2_spec.rb:3:in `<top (required)>'
[- | ==================100%================== | 00:00:00]      1F      0E

Finished in 103.288794 seconds

3607 files, 28545 examples, 208272 expectations, 1 failure, 0 errors, 0 tagged
uncommon.mk:777: recipe for target 'yes-test-spec' failed
make: *** [yes-test-spec] Error 1
```

Let me skip this for now and enable MJIT CI again.

Related to Bug#14867

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63876 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Reading win32/win32.c waitpid implementation, maybe waitpid(-1, ...)
on that platform will never conflict with mjit use of waitpid.

In any case, I've added WAITPID_USE_SIGCHLD macro to vm_core.h
so it can be easy for Linux/BSD users to test (hopefully!)
win32-compatible code.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63855 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

The change is unstable on Windows. Please re-commit it when it correctly
supports Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63852 b2dd03c8-39d4-4d8f-98ff-823fe69b080e