libaom: fix assembly sources for x86_64

BUG=1055917

Change-Id: Ic3e12c711c5470a1ebf09dd72df392d85d1226cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106557Reviewed-by: James Zern <jzern@google.com>
Reviewed-by: Tom Finegan <tomfinegan@chromium.org>
Commit-Queue: Jerome Jiang <jianj@google.com>
Cr-Commit-Position: refs/heads/master@{#751099}

third_party/libaom/BUILD.gn

@@ -75,11 +75,13 @@ if (enable_libaom) {
       sources += aom_dsp_common_asm_ssse3
       sources += aom_ports_asm_x86
       sources += aom_av1_encoder_asm_sse2
-      sources += aom_av1_encoder_asm_ssse3_x86_64
       sources += aom_dsp_encoder_asm_sse2
-      sources += aom_dsp_encoder_asm_sse2_x86_64
-      sources += aom_dsp_encoder_asm_ssse3_x86_64
-      sources += aom_dsp_encoder_avx_asm_x86_64
+      if (current_cpu == "x64") {
+        sources += aom_av1_encoder_asm_ssse3_x86_64
+        sources += aom_dsp_encoder_asm_sse2_x86_64
+        sources += aom_dsp_encoder_asm_ssse3_x86_64
+        sources += aom_dsp_encoder_avx_asm_x86_64
+      }
       defines = [ "CHROMIUM" ]
       include_dirs = libaom_include_dirs
     }