Land chromium-side work to clean up handling of v8_target_cpu in the GN build.

build/toolchain/linux/BUILD.gn

Index: build/toolchain/linux/BUILD.gn
diff --git a/build/toolchain/linux/BUILD.gn b/build/toolchain/linux/BUILD.gn
index 1fe3c7d526962c62faaaf30aec73387e15c1f62a..51e26cfce4d635a1317ee029503ccb2d44042b6a 100644
--- a/build/toolchain/linux/BUILD.gn
+++ b/build/toolchain/linux/BUILD.gn
@@ -38,6 +38,12 @@ clang_toolchain("clang_x86") {
   toolchain_os = "linux"
 }
 
+clang_toolchain("clang_x86_v8_arm") {
+  toolchain_cpu = "x86"
+  v8_toolchain_cpu = "arm"
+  toolchain_os = "linux"
+}
+
 gcc_toolchain("x86") {
   cc = "gcc"
   cxx = "g++"
@@ -57,6 +63,12 @@ clang_toolchain("clang_x64") {
   toolchain_os = "linux"
 }
 
+clang_toolchain("clang_x64_v8_arm64") {
+  toolchain_cpu = "x64"
+  v8_toolchain_cpu = "arm64"
+  toolchain_os = "linux"
+}
+
 gcc_toolchain("x64") {
   cc = "gcc"
   cxx = "g++"