floss: Move host build check to parent BUILD file

On cross-compiles, the main build was failing because //bt:tools was
dependending on //bt/system:tools which doesn't exist on cross-compile
builds. Thus, move the condition to //bt:tools instead.

Bug: 201687614
Tag: #floss
Test: emerge-arm-generic floss on ChromeOS
Change-Id: I915ef8f8fa80f4642972a9aec960b428c581360d
diff --git a/system/BUILD.gn b/system/BUILD.gn
index 266f0fc..490ceb7 100644
--- a/system/BUILD.gn
+++ b/system/BUILD.gn
@@ -53,13 +53,11 @@
   #}
 #}
 
-if (host_cpu == target_cpu && host_os == target_os) {
-  group("tools") {
-    deps = [
-      "//bt/system/gd/dumpsys/bundler:bluetooth_flatbuffer_bundler",
-      "//bt/system/gd/packet/parser:bluetooth_packetgen",
-    ]
-  }
+group("tools") {
+  deps = [
+    "//bt/system/gd/dumpsys/bundler:bluetooth_flatbuffer_bundler",
+    "//bt/system/gd/packet/parser:bluetooth_packetgen",
+  ]
 }
 
 if (defined(use.android) && use.android) {