This adds optional multi-architecture builds, allowing to compile
x86 and x64 in one build. The correctness fuzzer can be configured to
compare the two executables, e.g. to compare x86 to x64 run the
launcher with: --second-d8=clang_x86/d8 in an x64 build.

Configuring the executable's architecture is now simplified and
inferred from the gn build configuration.

Building for clusterfuzz has now a new canonical target that can be
used by the infrastructure (defaults to d8).

The clusterfuzz release builder is now defined to compile
multi-arch builds, which will have an effect as soon as the
infrastructure refers to the new clusterfuzz target.

BUG=chromium:673246
NOTRY=true
TBR=mstarzinger,jarin

Review-Url: https://codereview.chromium.org/2649133010
Cr-Commit-Position: refs/heads/master@{#42884}