Limit fx_libopenjpeg_warnings config.

The warnings only occur with MSVC for 32-bit builds.

Change-Id: I8cca6fe204bcf9d1e77cb97f62517e7f7407e3e2
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/69870
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index f123814..7d44e2d 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -390,7 +390,7 @@
 
 config("fx_libopenjpeg_warnings") {
   visibility = [ ":*" ]
-  if (is_win) {
+  if (is_win && !is_clang && target_cpu == "x86") {
     cflags = [
       # Signed/unsigned comparisons.
       "/wd4018",