[CMake][Fuchsia] Use RelWithDebInfo to build runtimes

We want to preserve debug info in our runtimes to aid symbolization and
debugging; for shared libraries this will be stripped away during
install-stripped step and distributed via .build-id, for static archives
it's part of the archive and it's a responsibility of the consumer to
strip it away in the final binary if not needed.

Differential Revision: https://reviews.llvm.org/D64605

llvm-svn: 365845