Fix the GN build for MIPS.

Define V8_TARGET_ARCH_MIPS or V8_TARGET_ARCH_MIPS64 if the target
arch is mipsel or mips64el.

R=dpranke@chromium.org,jochen@chromium.org,machenbach@chromium.org
BUG=v8:3972
LOG=N

Review URL: https://codereview.chromium.org/1016923002

Cr-Commit-Position: refs/heads/master@{#27249}

BUILD.gn

@@ -113,7 +113,7 @@ config("toolchain") {
   defines = []
   cflags = []
 
-  # TODO(jochen): Add support for arm subarchs, mips, mipsel.
+  # TODO(jochen): Add support for arm subarchs, mips, mipsel, mips64el.
 
   if (v8_target_arch == "arm") {
     defines += [ "V8_TARGET_ARCH_ARM" ]
@@ -150,6 +150,12 @@ config("toolchain") {
   if (v8_target_arch == "arm64") {
     defines += [ "V8_TARGET_ARCH_ARM64" ]
   }
+  if (v8_target_arch == "mipsel") {
+    defines += [ "V8_TARGET_ARCH_MIPS" ]
+  }
+  if (v8_target_arch == "mips64el") {
+    defines += [ "V8_TARGET_ARCH_MIPS64" ]
+  }
   if (v8_target_arch == "x86") {
     defines += [ "V8_TARGET_ARCH_IA32" ]
   }