Backed out 2 changesets (bug 1569728) for Android 4.0 API16+ GeckoView multi-arch fat AAR opt build bustages.

Backed out changeset a420714ee262 (bug 1569728)
Backed out changeset d24ed38c76cd (bug 1569728)

build/unix/stdc++compat/stdc++compat.cpp

@@ -16,7 +16,6 @@
    GLIBCXX_3.4.20 is from gcc 4.9.0 (199307)
    GLIBCXX_3.4.21 is from gcc 5.0 (210290)
    GLIBCXX_3.4.22 is from gcc 6.0 (222482)
-   GLIBCXX_3.4.23 is from gcc 7.0
 
 This file adds the necessary compatibility tricks to avoid symbols with
 version GLIBCXX_3.4.17 and bigger, keeping binary compatibility with
@@ -145,11 +144,3 @@ namespace std {
 template basic_ios<char, char_traits<char> >::operator bool() const;
 }  // namespace std
 #endif
-
-#if MOZ_LIBSTDCXX_VERSION >= GLIBCXX_VERSION(3, 4, 23)
-namespace std {
-/* Instantiate this template to avoid GLIBCXX_3.4.23 symbol versions
- * depending on optimization level */
-template basic_string<char, char_traits<char>, allocator<char>>::basic_string(const basic_string&, size_t, const allocator<char>&);
-} // namespace std
-#endif

taskcluster/ci/toolchain/clang.yml

@@ -26,7 +26,7 @@ linux64-clang-4.0:
         fetch:
             - clang-4.0
         toolchain:
-            - linux64-gcc-7
+            - linux64-gcc-6
 
 linux64-clang-7:
     description: "Clang 7 toolchain build"
@@ -44,7 +44,7 @@ linux64-clang-7:
         fetch:
             - clang-7
         toolchain:
-            - linux64-gcc-7
+            - linux64-gcc-6
 
 linux64-clang-8:
     description: "Clang 8 toolchain build"
@@ -63,7 +63,7 @@ linux64-clang-8:
         fetch:
             - clang-8
         toolchain:
-            - linux64-gcc-7
+            - linux64-gcc-6
 
 linux64-clang-8-mingw-x86:
     description: "MinGW-Clang Trunk x86 toolchain build"
@@ -85,7 +85,7 @@ linux64-clang-8-mingw-x86:
             - libunwind
             - llvm-mingw
         toolchain:
-            - linux64-gcc-7
+            - linux64-gcc-6
 
 linux64-clang-8-mingw-x64:
     description: "MinGW-Clang Trunk x64 toolchain build"
@@ -108,7 +108,7 @@ linux64-clang-8-mingw-x64:
             - libunwind
             - llvm-mingw
         toolchain:
-            - linux64-gcc-7
+            - linux64-gcc-6
 
 linux64-clang-8-android-cross:
     description: "Clang 8 toolchain build"
@@ -127,7 +127,7 @@ linux64-clang-8-android-cross:
         fetch:
             - clang-8
         toolchain:
-            - linux64-gcc-7
+            - linux64-gcc-6
             - linux64-android-ndk-linux-repack
 
 linux64-clang-8-aarch64-cross:
@@ -152,7 +152,7 @@ linux64-clang-8-aarch64-cross:
             - clang-8
         toolchain:
             - linux64-binutils
-            - linux64-gcc-7
+            - linux64-gcc-6
 
 linux64-clang-8-macosx-cross:
     description: "Clang 8 toolchain build with MacOS Compiler RT libs"
@@ -179,7 +179,7 @@ linux64-clang-8-macosx-cross:
         toolchain:
             - linux64-cctools-port
             - linux64-clang-8
-            - linux64-gcc-7
+            - linux64-gcc-6
 
 macosx64-clang:
     description: "Clang toolchain build"
@@ -208,7 +208,7 @@ macosx64-clang:
         toolchain:
             - linux64-cctools-port
             - linux64-clang-8
-            - linux64-gcc-7
+            - linux64-gcc-6
             - linux64-node
 
 win64-clang-cl: