Fix make jobserver for runtime_tests and friends

Summary:
Building `runtime_tests` was compiling a single file at a time, with
`make[1]: warning: jobserver unavailable: using -j1.  Add '+' to parent make
rule.` appearing in the output.

The [GNU make
manual](https://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html)
says that to propagate jobserver information, `$(MAKE)` must appear directly in
the recipe, but it won't be detected if the variable is referenced through a
substitution like these rules do since D50516544. Adding an explicit `+` to
each rule fixes it.

Reviewed By: mpage

Differential Revision: D52546490

fbshipit-source-id: bdd42641e884a14e82d463e03e0ccbfa90ca85a4

Makefile.pre.in

@@ -2538,23 +2538,23 @@ define RUNTIME_TEST_BODY
 endef
 
 runtime_tests: @DEF_MAKE_RULE@ platform $(JIT_HEADERS) python-config
-	$(call RUNTIME_TEST_BODY, runtime_tests)
+	+$(call RUNTIME_TEST_BODY, runtime_tests)
 .PHONY: runtime_tests
 
 testruntime: @DEF_MAKE_RULE@ platform $(JIT_HEADERS) python-config
-	$(call RUNTIME_TEST_BODY, testruntime)
+	+$(call RUNTIME_TEST_BODY, testruntime)
 .PHONY: testruntime
 
 testruntime_serial: @DEF_MAKE_RULE@ platform $(JIT_HEADERS) python-config
-	$(call RUNTIME_TEST_BODY, testruntime_serial)
+	+$(call RUNTIME_TEST_BODY, testruntime_serial)
 .PHONY: testruntime_serial
 
 strict_module_tests: @DEF_MAKE_RULE@ platform $(JIT_HEADERS) python-config
-	$(call RUNTIME_TEST_BODY, strict_module_tests)
+	+$(call RUNTIME_TEST_BODY, strict_module_tests)
 .PHONY: strict_module_tests
 
 test_strict_module: @DEF_MAKE_RULE@ platform $(JIT_HEADERS) python-config
-	$(call RUNTIME_TEST_BODY, test_strict_module)
+	+$(call RUNTIME_TEST_BODY, test_strict_module)
 .PHONY: test_strict_module