Config changes to support target_os="fuchsia"

Fuchsia support has been upstreamed into Clang, so the existing
Linux-hosted Clang toolchain binaries in third_party with the correct
--target are sufficient.  Fuchsia is POSIXy so a lot of things will just
work, but some places will require different implementations, so this CL
adds support for is_fuchsia as well as a basic implementation of the
toolchain config.

The SDK (headers, sysroot, etc.) is currently available at
go/fuchsia-sdk until a standard location for it is decided upon.

The near term goal behind this CL is to bring up base/ and net/ on an
fyi bot running Fuchsia.

BUG=706592

Review-Url: https://codereview.chromium.org/2784063002
Cr-Commit-Position: refs/heads/master@{#471360}