Add Loongson identification variables "_MIPS_ARCH_LOONGSON".

In order to distinguish the difference with other MIPS architectures.

BUG=nobug

R=dpranke@chromium.org

Change-Id: I9dd7232d7d552528edf39f035d83634a1a889270
Reviewed-on: https://chromium-review.googlesource.com/849706
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: 汪 清 <wangqing-hf@loongson.cn>
Cr-Original-Commit-Position: refs/heads/master@{#527206}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 04fa422f9f37766f505d4d3e5b9208b0b80a8c75

config/compiler/BUILD.gn

@@ -625,6 +625,7 @@ config("compiler") {
 config("compiler_cpu_abi") {
   cflags = []
   ldflags = []
+  defines = []
 
   if (is_posix && !(is_mac || is_ios)) {
     # CPU architecture. We may or may not be doing a cross compile now, so for
@@ -743,6 +744,7 @@ config("compiler_cpu_abi") {
           ]
         }
       } else if (mips_arch_variant == "loongson3") {
+        defines += [ "_MIPS_ARCH_LOONGSON" ]
         cflags += [
           "-march=loongson3a",
           "-mno-branch-likely",
@@ -849,6 +851,7 @@ config("compiler_cpu_abi") {
         ]
         ldflags += [ "-mips64r2" ]
       } else if (mips_arch_variant == "loongson3") {
+        defines += [ "_MIPS_ARCH_LOONGSON" ]
         cflags += [
           "-march=loongson3a",
           "-mno-branch-likely",