docs(build): Add a note for macOS

Update how-to-build.rst - Added another hint for macOS, for the common
problem that strip fails on mpy-cross with "malformed object (unknown
load command 9)"

Manual merge for !173

Documentation/how-to-build.rst

@@ -144,6 +144,15 @@ In order to do a rebuild you can issue a clean command to ninja via
 
 Otherwise, rerunning ``./bootstrap.sh`` will also clean the build-directory.
 
+.. note::
+
+   **macOS**: If ``strip`` fails to work on the freshly compiled ``mpy-cross``:
+   "strip: object: (...)/lib/micropython/micropython/mpy-cross/mpy-cross
+   malformed object (unknown load command 9)", you a likely not using the
+   `strip` that matches to your ``clang``. Do ``which strip && which clang``,
+   and if the paths don't match, clean up your PATHs, or as a quick hack,
+   create a symlink for strip.
+
 .. note::
 
    If you try to flash pycardium_epicardium.bin (renamed to card10.bin)