[Fuchsia] Use libc++abi on Windows in Fuchsia toolchain

[phosek]

Summary

Don't use vcruntime, this makes our toolchain more hermetic and avoids
some compiler errors we've encountered in compiler-rt.

[mcgrathr]

lgtm

clang/cmake/caches/Fuchsia-stage2.cmake

if(WIN32)
 
   list(APPEND BUILTIN_TARGETS "${target}")
   set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Windows CACHE STRING "")
   set(BUILTINS_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
 
   list(APPEND RUNTIME_TARGETS "${target}")
   set(RUNTIMES_${target}_CMAKE_SYSTEM_NAME Windows CACHE STRING "")
   set(RUNTIMES_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
+  set(RUNTIMES_${target}_LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
+  set(RUNTIMES_${target}_LIBCXX_CXX_ABI "libcxxabi" CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
-  set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx" CACHE STRING "")
+  set(RUNTIMES_${target}_LIBCXX_NO_VCRUNTIME ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi" CACHE STRING "")
 endif()
 
 foreach(target aarch64-unknown-linux-gnu;armv7-unknown-linux-gnueabihf;i386-unknown-linux-gnu;x86_64-unknown-linux-gnu)
   if(LINUX_${target}_SYSROOT)
     # Set the per-target builtins options.
     list(APPEND BUILTIN_TARGETS "${target}")
     set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")