[Bazel] Add support for lld

This patch adds a Bazel configuration to build lld. That includes a
BUILD.bazel file to export the libunwind headers for use by lld. Since
the lld target itself requires libxml2 (through WindowsManifest) it's
currently disabled on Buildkite and marked manual, but all the libraries
build.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D107414