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

BUILD.gn

@@ -24,6 +24,9 @@ group("all") {
   deps = [ "//bt/system:all" ]
 }
 
-group("tools") {
-  deps = [ "//bt/system:tools" ]
+# Tools should be built for the host target.
+if (host_cpu == target_cpu && host_os == target_os) {
+  group("tools") {
+    deps = [ "//bt/system:tools" ]
+  }
 }

system/BUILD.gn

@@ -53,13 +53,11 @@ group("bluetooth") {
   #}
 #}
 
-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) {