build: fix mkpeephole configuration

mkpeephole is a binary that gets generated and run at V8 build time.
On cross-compilation builds separate build toolchains are required.
Similar to want_separate_host_toolset we default to disabled.

PR-URL: https://github.com/nodejs/node/pull/8317


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michaël Zasso <targos@protonmail.com>