Because of the somewhat strange way default toolchains and custom
toolchains and user-specified arguments work in GN, if you did a v8
build that just set v8_target_cpu, you could end up building two
identical copies of v8 (see the comments in the change for more).

This CL identifies that case and fixes it.

R=machenbach@chromium.org
BUG=629825

Review-Url: https://codereview.chromium.org/2166173002
Cr-Commit-Position: refs/heads/master@{#37926}