[CMake] Update Fuchsia build configuration

This includes several changes to our toolchain build:
- Switch from RelWithDebInfo to Release as we don't currently use the
  debug info and therefore don't need to produce it.
- Disable unwind tables since we don't need them.
- Disable Clang static analyzer and ARCMT since we don't use it.
- Disable Go tests since we don't distribute Go bindings.
- Set the deployment target for macOS to 10.7 to make the toolchain
  usable on older systems.
- Restrict the targets we build runtimes for on Darwin only the ones
  we actually need.
- Drop llc and opt from the distribution since we don't need them.

Differential Revision: https://reviews.llvm.org/D69415