GN: mac host and armv7 target

Just when I thought it wouldn't be useful to override ar...

Tested by building 32- and 64-bit DM on my MBP and running it on my N5x.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2279703003

Review-Url: https://codereview.chromium.org/2279703003
diff --git a/BUILD.gn b/BUILD.gn
index 8c053cf..02cb350 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -215,6 +215,12 @@
   cflags = [ "-mavx" ]
 }
 
+opts("dsp") {
+  enabled = current_cpu == "mipsel"
+  sources = opts_gypi.mips_dsp_sources
+  cflags = [ "-Wno-deprecated-register" ]  # FIXME
+}
+
 # Any feature of Skia that requires third-party code should be optional and use this template.
 template("optional") {
   if (invoker.enabled) {
@@ -334,6 +340,7 @@
     ":armv7",
     ":avx",
     ":crc32",
+    ":dsp",
     ":gif",
     ":jpeg",
     ":none",