[gn] Support for building compiler-rt builtins

This is support for building compiler-rt builtins, The library build
should be complete for a subset of supported platforms, but not all
CMake options have been replicated in GN.

We always use the just built compiler to build all the runtimes, which
is equivalent to the CMake runtimes build. This simplifies the build
configuration because we don't need to support arbitrary host compiler
and can always assume the latest Clang. With GN's toolchain support,
this is significantly more efficient than the CMake runtimes build.

Differential Revision: https://reviews.llvm.org/D60331

llvm-svn: 357821

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