[CMake][Fuchsia] Drop Darwin architectures

[phosek]

Summary

We want to build runtimes for all available ones.

clang/cmake/caches/Fuchsia-stage2.cmake

if(APPLE)
   set(LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
   set(LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
   set(LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
   set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
   set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
   set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
   set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
-  set(DARWIN_ios_ARCHS arm64 CACHE STRING "")
-  set(DARWIN_iossim_ARCHS arm64 CACHE STRING "")
-  set(DARWIN_osx_ARCHS arm64;x86_64 CACHE STRING "")
 endif()
 
 if(WIN32)
   set(target "x86_64-pc-windows-msvc")
 
   list(APPEND BUILTIN_TARGETS "${target}")
   set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Windows CACHE STRING "")
   set(BUILTINS_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")