GN: Fix gn gen/compile errors for MIPS

Fixing two errors:
 - ERROR Unresolved dependencies.
   //:gn_only(//build/toolchain/linux:mipsel)
   needs //third_party/libjpeg_turbo:simd_asm(//build/toolchain/linux:mipsel)
 by including //third_party/libjpeg_turbo:simd_asm target
 only for x86 and x64, where it exists
 - incompatible pointer types in usrsctp
 by ignoring warnings for this module

TEST=gn gen out/mips-linux --args="is_debug=false target_os=\"linux\" target_cpu=\"mipsel\" is_clang=false"
     ninja -C out/mips-linux/ chrome

Review URL: https://codereview.chromium.org/1094793004

Cr-Commit-Position: refs/heads/master@{#327245}

BUILD.gn

@@ -603,7 +603,6 @@ group("gn_only") {
       "//third_party/pdfium/third_party:freetype",
       "//third_party/libjingle:peerconnnection_server",
       "//third_party/libjpeg_turbo:simd",
-      "//third_party/libjpeg_turbo:simd_asm",
       "//third_party/libsrtp:replay_driver",
       "//third_party/libsrtp:roc_driver",
       "//third_party/libsrtp:rtpw",
@@ -630,6 +629,9 @@ group("gn_only") {
       "//ui/shell_dialogs:shell_dialogs_unittests",
       "//ui/views/examples:views_examples_exe",
     ]
+    if (target_cpu == "x86" || target_cpu == "x64") {
+      deps += [ "//third_party/libjpeg_turbo:simd_asm" ]
+    }
     if (enable_nacl) {
       deps += [ "//native_client/src/trusted/service_runtime:sel_ldr" ]
     }

third_party/usrsctp/BUILD.gn

@@ -122,6 +122,11 @@ static_library("usrsctp") {
 
   if (is_clang) {
     cflags = [ "-Wno-incompatible-pointer-types" ]
+  } else if (target_cpu == "mipsel") {
+    # mipsel is still using gcc (4.9),
+    # which does not support "-Wno-incompatible-pointer-types"
+    # so let's disable all warnings for this module.
+    cflags = [ "-w" ]
   }
 
   deps = [