[CMake] Link static libunwind and libc++abi into libc++ in Fuchsia toolchain

[phosek]

Summary

When building libc++ for Fuchsia, we want to distribute shared libc++,
libc++abi and libunwind as separate libraries, but for static versions
we would like to link all of them into libc++ so -lc++ flag has the same
effect whether shared or static library is being used.

[jakehehrlich]

LGTM

clang/cmake/caches/Fuchsia-stage2.cmake

foreach(target x86_64;aarch64)
     set(RUNTIMES_${target}-fuchsia_CMAKE_SHARED_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE PATH "")
     set(RUNTIMES_${target}-fuchsia_CMAKE_MODULE_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE PATH "")
     set(RUNTIMES_${target}-fuchsia_CMAKE_EXE_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE PATH "")
     set(RUNTIMES_${target}-fuchsia_CMAKE_SYSROOT ${FUCHSIA_${target}_SYSROOT} CACHE PATH "")
     set(RUNTIMES_${target}-fuchsia_LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
+    set(RUNTIMES_${target}-fuchsia_LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
+    set(RUNTIMES_${target}-fuchsia_LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
+    set(RUNTIMES_${target}-fuchsia_LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
+    set(RUNTIMES_${target}-fuchsia_LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY OFF CACHE BOOL "")
   endforeach()
 
   set(LLVM_RUNTIME_SANITIZERS "Address" CACHE STRING "")
   set(LLVM_RUNTIME_SANITIZER_Address_TARGETS "x86_64-fuchsia;aarch64-fuchsia" CACHE STRING "")
 endif()
 
 set(LLVM_BUILTIN_TARGETS "${BUILTIN_TARGETS}" CACHE STRING "")
 set(LLVM_RUNTIME_TARGETS "${RUNTIME_TARGETS}" CACHE STRING "")