[CMake] Set CMAKE_BUILD_WITH_INSTALL_RPATH for Fuchsia runtimes

[phosek]

Summary

This doesn't make any difference since we don't use RPATH/RUNPATH
on Fuchsia but it avoids the CMake error when re-linking libraries
while building with Ninja.

[phosek]

Actually after testing this I believe this is really what we want, without this option CMake sets RPATH for build libraries and then removes it when installing these. With this option it doesn't even set the RPATH for build libraries.

[jakehehrlich]

Need some clarification on this option

1. The rpath is the path from which libs mentioned in .dynamic are relative to?
2. Normally cmake first links binaries without setting the RPATH and then sets it later.
3. This makes RPATH always equal to the empty string and avoids the "relinking" step that resets RPATH

If so this LGTM

[jakehehrlich]

Spoke offline, LGTM

clang/cmake/caches/Fuchsia-stage2.cmake

if(NOT APPLE)
   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_${target}-fuchsia_CMAKE_SYSTEM_NAME Fuchsia CACHE STRING "")
   set(RUNTIMES_${target}-fuchsia_CMAKE_BUILD_TYPE ${FUCHSIA_RUNTIMES_BUILD_TYPE} CACHE STRING "")
+  set(RUNTIMES_${target}-fuchsia_CMAKE_BUILD_WITH_INSTALL_RPATH ON CACHE STRING "")
   set(RUNTIMES_${target}-fuchsia_CMAKE_ASM_FLAGS ${FUCHSIA_${target}_C_FLAGS} CACHE PATH "")
   set(RUNTIMES_${target}-fuchsia_CMAKE_C_FLAGS ${FUCHSIA_${target}_C_FLAGS} CACHE PATH "")
   set(RUNTIMES_${target}-fuchsia_CMAKE_CXX_FLAGS ${FUCHSIA_${target}_CXX_FLAGS} CACHE PATH "")
   set(RUNTIMES_${target}-fuchsia_CMAKE_EXE_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE PATH "")
   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_SYSROOT ${FUCHSIA_${target}_SYSROOT} CACHE PATH "")
   set(RUNTIMES_${target}-fuchsia_LLVM_ENABLE_ASSERTIONS ${FUCHSIA_RUNTIMES_ENABLE_ASSERTIONS} CACHE BOOL "")