Do not depend on yasm for non-Fuchsia targets

Change-Id: I68cc4b4aca20874271b5b259aa341101a2c4b9e0
diff --git a/BUILD.gn b/BUILD.gn
index f7093bb..7dd55c3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,7 +8,7 @@
   import("//build/config/arm.gni")
 }
 
-if (current_cpu == "x86" || current_cpu == "x64") {
+if ((current_cpu == "x86" || current_cpu == "x64") && is_fuchsia) {
   import("//third_party/yasm/yasm_assemble.gni")
 
   yasm_assemble("simd_asm") {