[infra] Only enabling SSE2 on MSVC builds with 32-bit CPU.

Bug: pdfium:1714
Change-Id: Idde56691ca9dc12bd14107c57b0865faa60fb728
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/84810
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 94edcde..885c2bb 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -43,7 +43,7 @@
   if (is_win) {
     # Assume UTF-8 by default to avoid code page dependencies.
     cflags += [ "/utf-8" ]
-    if (msvc_use_sse2 && !is_clang) {
+    if (msvc_use_sse2 && !is_clang && current_cpu == "x86") {
       cflags += [ "/arch:SSE2" ]
     }
   }