GN: support 32-bit x86 builds

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 05fe3500ffdaa5335346ef07fc57f55144008bfe..46d443474f2fde881c22cdc9751f4e3f02a8e7ab 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -554,22 +554,26 @@ action("skia.h") {
   ]
 }
 
-executable("fiddle") {
-  include_dirs = [ "$target_gen_dir" ]
-  libs = []
-  if (is_linux) {
-    libs += [ "OSMesa" ]
-  }
+if (target_cpu == "x64") {
+  # Our bots only have 64-bit libOSMesa installed.
+  # TODO: worth fixing?
+  executable("fiddle") {
+    include_dirs = [ "$target_gen_dir" ]
+    libs = []
+    if (is_linux) {
+      libs += [ "OSMesa" ]
+    }
 
-  sources = [
-    "src/images/SkForceLinking.cpp",
-    "tools/fiddle/draw.cpp",
-    "tools/fiddle/fiddle_main.cpp",
-  ]
-  deps = [
-    ":skia",
-    ":skia.h",
-  ]
+    sources = [
+      "src/images/SkForceLinking.cpp",
+      "tools/fiddle/draw.cpp",
+      "tools/fiddle/fiddle_main.cpp",
+    ]
+    deps = [
+      ":skia",
+      ":skia.h",
+    ]
+  }
 }
 
 # Targets guarded by skia_enable_tools may use //third_party freely.