libyuv:Add MultiMedia Instruction(MMI) build flags for GYP/GN builds

A preparation patch for adding MMI optimizations in libYUV project.

BUG=libyuv:804

Review-Url: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/1122064
Change-Id: I8840920f628095784756b5b3791bae47197e0a5a
Reviewed-on: https://chromium-review.googlesource.com/1137811Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576496}

build/config/mips.gni

@@ -8,6 +8,11 @@ import("//build/config/v8_target_cpu.gni")
 # MIPS code is being compiled.  But they can also be relevant in the
 # other contexts when the code will change its behavior based on the
 # cpu it wants to generate code for.
+declare_args() {
+  # MIPS MultiMedia Instruction compilation flag.
+  mips_use_mmi = false
+}
+
 if (current_cpu == "mipsel" || v8_current_cpu == "mipsel" ||
     current_cpu == "mips" || v8_current_cpu == "mips") {
   declare_args() {