We need the v8 snapshot to be compiled on the host cpu (the
machine doing the build), but using generated code that has the
same pointer size as the target_arch; i.e., for 32-bit arm builds,
we need to use a 32-bit x86 host binary, not a 64-bit host binary.

The easiest way to ensure that this happens is to just specify
a custom toolchain in GN to use to build the snapshot.

R=jochen@chromium.org, cjhopman@chromium.org
BUG=465456, 395249
LOG=Y

Review URL: https://codereview.chromium.org/993173003

Cr-Commit-Position: refs/heads/master@{#27142}