[Linux][Build] Use host swiftrt.o when in HOSTTOOLS mode.

When we're building with host tools, we should use the host's swiftrt.o
and not the one we've just built when we're trying to build tools that we
will run on the host system.

rdar://115774613

Author: al45tair

cmake/modules/AddSwift.cmake

@@ -540,6 +540,7 @@ function(_add_swift_runtime_link_flags target relpath_to_lib_dir bootstrapping)
       get_filename_component(swift_bin_dir ${SWIFT_EXEC_FOR_SWIFT_MODULES} DIRECTORY)
       get_filename_component(swift_dir ${swift_bin_dir} DIRECTORY)
       set(host_lib_dir "${swift_dir}/lib/swift/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_LIB_SUBDIR}")
+      set(swiftrt "${host_lib_dir}/${SWIFT_HOST_VARIANT_ARCH}/swiftrt.o")
 
       target_link_libraries(${target} PRIVATE ${swiftrt})
       target_link_libraries(${target} PRIVATE "swiftCore")