compiler bootstrapping: enable both single-node and multi-node DAGS(#12310)

5f74f22 enabled installing compilers for dependencies but not for the root package (and in particular not for DAGs which consist of one package)

this enables bootstrapping compilers for both types of DAGs

lib/spack/spack/package.py

@@ -1534,15 +1534,13 @@ def do_install(self, **kwargs):
             dep_kwargs['install_deps'] = False
             for dep in self.spec.traverse(order='post', root=False):
                 if spack.config.get('config:install_missing_compilers', False):
-                    tty.debug('Bootstrapping {0} compiler for {1}'.format(
-                        self.spec.compiler, self.name
-                    ))
-                    comp_kwargs = kwargs.copy()
-                    comp_kwargs['explicit'] = False
-                    comp_kwargs['install_deps'] = True
-                    dep.package.bootstrap_compiler(**comp_kwargs)
+                    Package._install_bootstrap_compiler(dep.package, **kwargs)
                 dep.package.do_install(**dep_kwargs)
 
+        # Then, install the compiler if it is not already installed.
+        if install_deps:
+            Package._install_bootstrap_compiler(self, **kwargs)
+
         # Then, install the package proper
         tty.msg(colorize('@*{Installing} @*g{%s}' % self.name))
 
@@ -1705,6 +1703,16 @@ def build_process():
             # check the filesystem for it.
             self.stage.created = False
 
+    @staticmethod
+    def _install_bootstrap_compiler(pkg, **install_kwargs):
+        tty.debug('Bootstrapping {0} compiler for {1}'.format(
+            pkg.spec.compiler, pkg.name
+        ))
+        comp_kwargs = install_kwargs.copy()
+        comp_kwargs['explicit'] = False
+        comp_kwargs['install_deps'] = True
+        pkg.bootstrap_compiler(**comp_kwargs)
+
     def unit_test_check(self):
         """Hook for unit tests to assert things about package internals.