Both MSVC and Clang require a mainCRTStartup symbol for the cctest
executable to compile. All objects from the cctest_sources source
set are bundled into a library which does not contain this symbol.

Bug: v8:7854
Change-Id: I88cd26209114daa84574e3b20046613b1560fa98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1357039
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61206}