[MIPS64] Start using Clang IAS for mips64 Android

MIPS64 requires IAS to be explicitly set.

Few important mips64 related changes were missing in Clang.
With Clang roll to r295762 they are included and
clang does not crash anymore.

Now that build is ok we can switch to IAS as well.

BUG=None.
TEST=gn gen out-gn/mips64-clang --args="is_debug=false target_os=\"android\" target_cpu=\"mips64el\" is_clang=true mips_use_msa=false"
     ninja -C out-gn/mips64-clang system_webview_apk

Review-Url: https://codereview.chromium.org/2720833002
Cr-Original-Commit-Position: refs/heads/master@{#453252}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 55850d4e8fca1b77851f90956fe3dd995b01afc0

config/android/BUILD.gn

@@ -27,14 +27,10 @@ config("compiler") {
   ]
 
   if (is_clang) {
-    rebased_android_toolchain_root =
-        rebase_path(android_toolchain_root, root_build_dir)
-    assert(rebased_android_toolchain_root != "")  # Mark as used.
     if (current_cpu == "mips64el") {
       cflags += [
-        # TODO(gordanac) Enable integrated-as.
-        "-fno-integrated-as",
-        "-B${rebased_android_toolchain_root}/bin",  # Else /usr/bin/as gets picked up.
+        # Have to force IAS for mips64.
+        "-fintegrated-as",
       ]
     }
   } else {