libcxx: Fix linking based upon chosen compiler runtime

libcxx can use either libgcc_s or llvm libunwind for unwinder
functionality, the defaults are platform dependent, therefore with this
patch we make intentional choice based on RUNTIME settings chosen, e.g.
when using GNU runtime we use libgcc and also ensure that it add right
compiler options to build as well, since libcxx build looks C runtime by
calling $($CC --print-libgcc-file-name) to build libc++abi.so unless we
use right -rtlib option it will result in linking wrong library

Enable RTTI as well

do not enable llvm unwinder unless we chose llvm runtime, even that
might not work since libpthread from glibc explicitly dlopens
libgcc_s.so to enable unwinding

https://github.com/bminor/glibc/blob/5f72f9800b250410cad3abfeeb09469ef12b2438/sysdeps/nptl/unwind-forcedunwind.c#L53...L58

Fixes Issue #332

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

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