Run rb_syswait on exec failure

not only when !w but also when w == WAITPID_LOCK_ONLY.
See also: f7c0cc3692 and a226434206.
We thought this change was an oversight in the latter commit.

Without this change, the test fails like:

$ make test-all TESTS="../test/ruby/test_process.rb -n test_exec_failure_leaves_no_child" RUN_OPTS="--jit"
...
  1) Failure:
TestProcess#test_exec_failure_leaves_no_child [/home/k0kubun/src/github.com/ruby/ruby/test/ruby/test_process.rb:2493]:
Expected [[26799, #<Process::Status: pid 26799 exit 127>]] to be empty.

Co-Authored-By: Yusuke Endoh <mame@ruby-lang.org>

process.c

@@ -4046,7 +4046,7 @@ fork_check_err(int *status, int (*chfunc)(void*, char *, size_t), void *charg,
                       "only used by extensions");
             rb_protect(proc_syswait, (VALUE)pid, status);
         }
-        else if (!w) {
+        else if (!w || w == WAITPID_LOCK_ONLY) {
             rb_syswait(pid);
         }
         errno = err;

test/ruby/test_process.rb

@@ -2485,4 +2485,11 @@ EOS
     Process.wait spawn(RUBY, "-e", "exit 13")
     assert_same(Process.last_status, $?)
   end
+
+  def test_exec_failure_leaves_no_child
+    assert_raise(Errno::ENOENT) do
+      spawn('inexistent_command')
+    end
+    assert_empty(Process.waitall)
+  end
 end

test/rubygems/test_gem_ext_cmake_builder.rb

@@ -14,14 +14,6 @@ class TestGemExtCmakeBuilder < Gem::TestCase
       _, status = Open3.capture2e('cmake')
       skip 'cmake not present' unless status.success?
     rescue Errno::ENOENT
-      # Open3.capture2e with ENOENT with JIT enabled leaves a zombie process.
-      # TODO: avoid making the zombie on ENOENT with JIT
-      if defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled?
-        begin
-          Process.waitall
-        rescue Errno::ECHILD
-        end
-      end
       skip 'cmake not present'
     end