Revert of Fix double-building of v8 in GN builds when setting just v8_target_cpu. (patchset #1 id:1 of https://codereview.chromium.org/2166173002/ )

Reason for revert:
Breaks:
https://build.chromium.org/p/client.v8.fyi/builders/V8%20Android%20GN%20%28dbg%29/builds/4590

And also the trybot:
https://build.chromium.org/p/tryserver.chromium.android/builders/android_clang_dbg_recipe/builds/99806

Original issue's description:
> Fix double-building of v8 in GN builds when setting just v8_target_cpu.
>
> 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
>
> Committed: https://crrev.com/3536db45c9409c9aadc4eee6004cf337c0588cdb
> Cr-Commit-Position: refs/heads/master@{#37926}

TBR=dpranke@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=629825

Review-Url: https://codereview.chromium.org/2167113002
Cr-Commit-Position: refs/heads/master@{#37936}