Fix the toolchain used to build the snapshots in GN.

Fix the toolchain used to build the snapshots in GN.

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}