[Fuchsia] Don't install libc++, libc++abi or libunwind on Darwin

[phosek]

Summary

The Clang driver doesn't currently know how to use the libraries
that are shipped as part of the toolchain so there's no reason to
ship them at all.

[mcgrathr]

When targetting Darwin we should really not be even compiling libc++/libc++abi and certainly should not have anything else compiling against their headers since at runtime the system libraries will be used and so things better compile against the system headers that match.
Do these settings accomplish that?

[phosek]

Done, they're no longer installed on Darwin.

[mcgrathr]

Lgtm with some more comments

clang/cmake/caches/Fuchsia-stage2.cmake

 
 set(LLVM_RUNTIME_TARGETS "default;x86_64-fuchsia;aarch64-fuchsia;x86_64-fuchsia-asan:x86_64-fuchsia;aarch64-fuchsia-asan:aarch64-fuchsia" CACHE STRING "")
 
 # Set the default target runtimes options.
 set(LIBUNWIND_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
 set(LIBUNWIND_INSTALL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
-set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
-set(LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
+if(NOT APPLE)
+  set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
+  set(LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
+endif()
 
 # Set the per-target runtimes options.
 foreach(target x86_64;aarch64)
   set(RUNTIMES_${target}-fuchsia_CMAKE_BUILD_WITH_INSTALL_RPATH ON CACHE BOOL "")
   set(RUNTIMES_${target}-fuchsia_CMAKE_SYSROOT ${FUCHSIA_${target}_SYSROOT} CACHE PATH "")
   set(RUNTIMES_${target}-fuchsia_CMAKE_SYSTEM_NAME Fuchsia CACHE STRING "")
   set(RUNTIMES_${target}-fuchsia_UNIX 1 CACHE BOOL "")
   set(RUNTIMES_${target}-fuchsia_LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")