libcxx/compiler-rt: Add support for llvm-libgcc

libcxx/compiler-rt: Add support for llvm-libgcc

llvm-libgcc is a drop-in replacement for libgcc and crt* files
and its API compatible as well, built out of compiler-rt
and libunwind projects.

Its only enabled when TC_CXX_RUNTIME is set to 'llvm'
since user intends to use LLVM provided runtime packages
as default platform/system runtime.

Move libunwind out of libcxx recipe into compiler-rt
recipe, to facilicate building llvm-libgcc

Package and build crt* files, creating symlinks for
names as they are expected by gcc, to help build packages
with gcc when and where needed.

Always use LLD linker to build compiler-rt since the
versioning script to generate libgcc compatibility
expects it to be LLD and not BFD linker doing the link

Signed-off-by: Khem Raj <raj.khem@gmail.com>