Fix toolchain_cpu for clang_x64 android toolchain

Similar to https://dart-review.googlesource.com/c/sdk/+/396260, it
looks like the value for `android_app_abi` was used for `cpu`, but
before https://dart-review.googlesource.com/c/sdk/+/392661 `current_cpu`
wasn't actually set to `toolchain_cpu`.

Fixes #59621

Change-Id: Icf3a36c2b6d58c5979bb71ea67a864b658abc514
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397901
Reviewed-by: Slava Egorov <[email protected]>
Commit-Queue: Ivan Inozemtsev <[email protected]>

build/toolchain/android/BUILD.gn

@@ -20,7 +20,7 @@ android_toolchain_suite("clang_arm") {
 }
 
 android_toolchain_suite("clang_x64") {
-  toolchain_cpu = "x86_64"
+  toolchain_cpu = "x64"
 }
 
 android_toolchain_suite("clang_arm64") {