Change ROAR to be enabled by constraints

Summary:
ROAR is moving to use its own toolchain.  Change the flags to be injected by
constraints from build modes.  For link groups we have to override things
during the configuration phase since selects don't allow for returning a
defacto yes/no link groups are supported or not.

Reviewed By: ottoni

Differential Revision: D68218288

fbshipit-source-id: 2496a92ce970482614660a1a4b54f45b34e4a313

build_defs/roar.bzl

@@ -5,10 +5,8 @@
 # License, Version 2.0 found in the LICENSE-APACHE file in the root directory
 # of this source tree.
 
-load("@fbsource//tools/build_defs:buckconfig.bzl", "read_bool")
-
 def roar_no_jit():
-    use_roar_jit = read_bool("fbcode", "use_roar_jit", required = False)
-    if use_roar_jit:
-        return ["-fforce-no-jit"]
-    return []
+    return select({
+        "DEFAULT": [],
+        "ovr_config//third-party/llvm-fb/constraints:roar": ["-fforce-no-jit"],
+    })