Fix bug in `darwin_test_archs()` when the cache variable is set but empty.

[delcypher]

Summary

If the cache variable named in ${valid_archs} (e.g. DARWIN_osx_BUILTIN_ARCHS)
is set in the cache but is empty then the cache check
if(${valid_archs}) will be false so the function will probe the
compiler but the set(...) command at the end of the function to update
the cache variable will be a no-op. This is because set(...) will not
update an existing cache variable unless the FORCE argument is
provided.

To fix this this patch adds FORCE so the cache is always updated.

rdar://problem/55323665

[vsk]

Wow, thanks for chasing this down.

[delcypher]

@vsk Thanks for the review.

compiler-rt/trunk/cmake/Modules/CompilerRTDarwinUtils.cmake

if(${CAN_TARGET_${os}_${arch}})
       list(APPEND working_archs ${arch})
     else()
       file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
         "Testing compiler for supporting ${os}-${arch}:\n"
         "${TEST_OUTPUT}\n")
     endif()
   endforeach()
   set(${valid_archs} ${working_archs}
-    CACHE STRING "List of valid architectures for platform ${os}.")
+    CACHE STRING "List of valid architectures for platform ${os}." FORCE)
 endfunction()
 
 # This function checks the host cpusubtype to see if it is post-haswell. Haswell
 # and later machines can run x86_64h binaries. Haswell is cpusubtype 8.
 function(darwin_filter_host_archs input output)
   list_intersect(tmp_var DARWIN_osx_ARCHS ${input})
   execute_process(
     COMMAND sysctl hw.cpusubtype