Fix the v8 snapshots in the GN mipsel, mips64el builds.

This also deletes references to gcc-based snapshot toolchains that
don't actually exist. Let's find out if they're actually needed anywhere ;).
The mips build was broken by the v8_target_cpu changes for 625353.

TBR=machenbach@chromium.org
BUG=625353, 629057

Review-Url: https://codereview.chromium.org/2159003002
Cr-Original-Commit-Position: refs/heads/master@{#406174}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fb1cadc07b5b8f97f1ef5ee9800a47ce6f6fe902

config/v8_target_cpu.gni

@@ -34,12 +34,16 @@ declare_args() {
 }
 
 if (v8_target_cpu == "") {
-  if (current_toolchain == "//build/toolchain/linux:clang_x64_v8_arm64" ||
-      current_toolchain == "//build/toolchain/linux:x64_v8_arm64") {
+  if (current_toolchain == "//build/toolchain/linux:clang_x64_v8_arm64") {
     v8_target_cpu = "arm64"
-  } else if (current_toolchain == "//build/toolchain/linux:clang_x86_v8_arm" ||
-             current_toolchain == "//build/toolchain/linux:x86_v8_arm") {
+  } else if (current_toolchain == "//build/toolchain/linux:clang_x86_v8_arm") {
     v8_target_cpu = "arm"
+  } else if (current_toolchain ==
+             "//build/toolchain/linux:clang_x86_v8_mips64el") {
+    v8_target_cpu = "mips64el"
+  } else if (current_toolchain ==
+             "//build/toolchain/linux:clang_x86_v8_mipsel") {
+    v8_target_cpu = "mipsel"
   } else if (is_msan) {
     # If we're running under a sanitizer, if we configure v8 to generate
     # code that will be run under a simulator, then the generated code

toolchain/linux/BUILD.gn

@@ -44,6 +44,12 @@ clang_toolchain("clang_x86_v8_arm") {
   toolchain_os = "linux"
 }
 
+clang_toolchain("clang_x86_v8_mipsel") {
+  toolchain_cpu = "x86"
+  v8_toolchain_cpu = "mipsel"
+  toolchain_os = "linux"
+}
+
 gcc_toolchain("x86") {
   cc = "gcc"
   cxx = "g++"
@@ -69,6 +75,12 @@ clang_toolchain("clang_x64_v8_arm64") {
   toolchain_os = "linux"
 }
 
+clang_toolchain("clang_x64_v8_mips64el") {
+  toolchain_cpu = "x64"
+  v8_toolchain_cpu = "mips64el"
+  toolchain_os = "linux"
+}
+
 gcc_toolchain("x64") {
   cc = "gcc"
   cxx = "g++"