Kernel: Use our toolchain's c++filt tool for the kernel map

The host's version of c++filt might not work on some operating systems,
e.g. macOS.

CMakeLists.txt

@@ -175,6 +175,7 @@ else()
     set(CMAKE_STRIP ${TOOLCHAIN_PREFIX}strip)
     set(CMAKE_AR ${TOOLCHAIN_PREFIX}gcc-ar)
     set(CMAKE_OBJCOPY ${TOOLCHAIN_PREFIX}objcopy)
+    set(CMAKE_CXXFILT ${TOOLCHAIN_PREFIX}c++filt)
 endif()
 
 foreach(lang ASM C CXX OBJC OBJCXX)