(cherry picked from commit 19b4e0a)

Having

```nix
host_toolchain = "//build/toolchain/linux/unbundle:host";
v8_snapshot_toolchain = "//build/toolchain/linux/unbundle:host";
```

on native, non-cross-compilation builds roughly doubles the build steps
and, by proxy, compute and time needed to build.

So to resolve this, we conditionally change those values depending on
whether we are cross-compiling or not.

Co-authored-by: Adam Joseph <[email protected]>
(cherry picked from commit 5dec2c9)