Fix LDC compilation with LDC 1.37 universal on macOS. Make ExtractDMD…

…SystemLinker.cmake compatible with `-target` and `-arch` (#4431)

The attempt to use CMake's `target_link_options` for `D_LINKER_ARGS` failed, because the list can be a mix of library dependencies and options.

cmake/Modules/ExtractDMDSystemLinker.cmake

@@ -64,6 +64,11 @@ separate_arguments(linker_line)
 list(GET linker_line 0 D_LINKER_COMMAND)
 list(REMOVE_AT linker_line 0)
 
+# Fixup known flags with spaces, such as "-target triple" argument, which would be turned
+# into "-target;triple" by `separate_arguments`. Repair by merging into one list item.
+string(REGEX REPLACE ";-target;([A-Za-z0-9_-]+);" ";-target \\1;" linker_line "${linker_line}")
+string(REGEX REPLACE ";-arch;([A-Za-z0-9_-]+);" ";-arch \\1;" linker_line "${linker_line}")
+
 if("${D_COMPILER_ID}" STREQUAL "GDMD")
     # Filter linker arguments for those we know can be safely reused
     set(D_LINKER_ARGS)