Do not depend on yasm on Fuchsia SDK builds.

This extends the same convention currently used for all non-fuchsia targets to
Fuchsia targets with SDK builds.

Change-Id: I7f41c36a33f89ea107c6bc6e65d713877a072ea4

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 7dd55c3..3e9ced0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,11 +8,13 @@
   import("//build/config/arm.gni")
 }
 
-if ((current_cpu == "x86" || current_cpu == "x64") && is_fuchsia) {
+import("//build/fuchsia/sdk.gni")
+
+if ((current_cpu == "x86" || current_cpu == "x64") && is_fuchsia &&
+    !using_fuchsia_sdk) {
   import("//third_party/yasm/yasm_assemble.gni")
 
   yasm_assemble("simd_asm") {
-
     defines = []
 
     if (current_cpu == "x86") {
@@ -88,7 +90,7 @@
 }
 
 static_library("simd") {
-  if (!is_fuchsia) {
+  if (!is_fuchsia || using_fuchsia_sdk) {
     sources = [
       "jsimd_none.c",
     ]