[CMake] Support platform building builtins without a full toolchain
This patch adds support for building lib/builtins without a fully functioning toolchain. It allows you to bootstrap a cross-compiler, which previously couldn't be done with CMake.
This patch contains the following specific changes:
- Split builtin-specific code out of config-ix.cmake into builtin-config-ix.cmake
- Split some common CMake functionality needed by both builtins and sanitizers into base-config-ix.cmake
- Made lib/builtins/CMakeLists.txt able to be a top-level CMake configuration
I have tested this on Darwin targeting embedded Darwin, and on FreeBSD x86_64 targeting FreeBSD AArch64.
Reviewers: samsonov, iains, jroelofs
Subscribers: compnerd, aemerson, tberghammer, danalbert, srhines, emaste, llvm-commits
Differential Revision: http://reviews.llvm.org/D19742