Revert "Delete the v8_target_cpu/v8_current_cpu hack in the GN build."

This reverts commit d3e00ac37e3722cc93d102164b23629e8cc6af31.
ChromeOS would crash at boot and could not reach login screen with
commit d3e00a.

BUG=chromium:593461
TBR=dpranke@chromium.org
TBR=machenbach@chromium.org

Review-Url: https://codereview.chromium.org/2155183002
Cr-Original-Commit-Position: refs/heads/master@{#405989}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9557ecb972e31f33470584e31ffbd602c767913a

toolchain/gcc_toolchain.gni

@@ -475,6 +475,12 @@ template("gcc_toolchain") {
         v8_current_cpu = current_cpu
       }
 
+      # TODO(crbug.com/625353) - Delete after v8 has been updated
+      # to only refer to v8_current_cpu. Until then, we need to make
+      # sure that v8_current_cpu always has the same value as v8_target_cpu,
+      # so that //build defines evaluate the way v8 is expecting them to.
+      v8_current_cpu = v8_target_cpu
+
       # Disable sanitizers for non-default toolchains.
       is_asan = false
       is_cfi = false