[CMake] Don't build Linux targets on Darwin in Fuchsia toolchain

[phosek]

Summary

This is currently breaking because Linux target sysroots rely on
case sensitive filesystem which is not by default enabled on macOS.

[juliehockett]

LGTM

cmake/caches/Fuchsia-stage2.cmake

 set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
 set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
 set(CMAKE_C_FLAGS_RELWITHDEBINFO "-O3 -gline-tables-only -DNDEBUG" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O3 -gline-tables-only -DNDEBUG" CACHE STRING "")
 
 if(APPLE)
   list(APPEND BUILTIN_TARGETS "default")
   list(APPEND RUNTIME_TARGETS "default")
-endif()
-
+elseif(UNIX)
   foreach(target i386;x86_64;armhf;aarch64)
     if(LINUX_${target}_SYSROOT)
       # Set the per-target builtins options.
       list(APPEND BUILTIN_TARGETS "${target}-linux-gnu")
       set(BUILTINS_${target}-linux-gnu_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
       set(BUILTINS_${target}-linux-gnu_CMAKE_BUILD_TYPE Release CACHE STRING "")
       set(BUILTINS_${target}-linux-gnu_CMAKE_SYSROOT ${LINUX_${target}_SYSROOT} CACHE STRING "")
 
       # Set the per-target runtimes options.
       list(APPEND RUNTIME_TARGETS "${target}-linux-gnu")
       set(RUNTIMES_${target}-linux-gnu_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
       set(RUNTIMES_${target}-linux-gnu_CMAKE_BUILD_TYPE Release CACHE STRING "")
       set(RUNTIMES_${target}-linux-gnu_CMAKE_SYSROOT ${LINUX_${target}_SYSROOT} CACHE STRING "")
       set(RUNTIMES_${target}-linux-gnu_LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBUNWIND_ENABLE_SHARED OFF CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBUNWIND_INSTALL_LIBRARY OFF CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
       set(RUNTIMES_${target}-linux-gnu_LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
     endif()
   endforeach()
+endif()
 
 if(FUCHSIA_SDK)
   set(FUCHSIA_aarch64_NAME arm64)
   set(FUCHSIA_x86_64_NAME x64)
   foreach(target x86_64;aarch64)
     set(FUCHSIA_${target}_COMPILER_FLAGS "-I${FUCHSIA_SDK}/pkg/fdio/include")
     set(FUCHSIA_${target}_LINKER_FLAGS "-L${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/lib")
     set(FUCHSIA_${target}_SYSROOT "${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/sysroot")

cmake/caches/Fuchsia.cmake

 
 set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
 
 if(APPLE)
   set(COMPILER_RT_ENABLE_IOS OFF CACHE BOOL "")
   set(COMPILER_RT_ENABLE_TVOS OFF CACHE BOOL "")
   set(COMPILER_RT_ENABLE_WATCHOS OFF CACHE BOOL "")
-else()
+elseif(UNIX)
   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 "")