tools/mpy_ld.py: Write architecture flags to output natmod if needed.

This commit lets "tools/mpy_ld.py" store architecture flags in generated
MPY files if explicitly requested, like "mpy-cross" does.

To achieve this, a new command-line option ("--arch-flags") was added to
receive the architecture flags value, accepting the same arguments'
format as "mpy-cross", and performing the same input validation.

The rest of the MPY toolchain was also modified to let the user pass the
arch flags to standard native module makefiles.  Given that there's
already a well-established "ARCH" argument, "ARCH_FLAGS" was chosen to
pass the optional flags to "mpy_ld.py".

Finally, documentation was updated to mention the new variable.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

docs/develop/natmod.rst

@@ -43,9 +43,14 @@ options for the ``ARCH`` variable, see below):
 * ``rv32imc`` (RISC-V 32 bits with compressed instructions, eg ESP32C3, ESP32C6)
 * ``rv64imc`` (RISC-V 64 bits with compressed instructions)
 
+If the chosen platform supports explicit architecture flags and you want to let
+the output .mpy file carry those flags' value, you must pass them to the
+``ARCH_FLAGS`` flags variable when building the .mpy file.
+
 When compiling and linking the native .mpy file the architecture must be chosen
-and the corresponding file can only be imported on that architecture.  For more
-details about .mpy files see :ref:`mpy_files`.
+and the corresponding file can only be imported on that architecture (and if
+architecture flags are present, only if they match the target's capabilities).
+For more details about .mpy files see :ref:`mpy_files`.
 
 Native code must be compiled as position independent code (PIC) and use a global
 offset table (GOT), although the details of this varies from architecture to
@@ -124,7 +129,8 @@ The filesystem layout consists of two main parts, the source files and the Makef
   location of the MicroPython repository (to find header files, the relevant Makefile
   fragment, and the ``mpy_ld.py`` tool), ``MOD`` as the name of the module, ``SRC``
   as the list of source files, optionally specify the machine architecture via ``ARCH``,
-  and then include ``py/dynruntime.mk``.
+  along with optional machine architecture flags specified via ``ARCH_FLAGS``, and
+  then include ``py/dynruntime.mk``.
 
 Minimal example
 ---------------
@@ -217,6 +223,10 @@ Without modifying the Makefile you can specify the target architecture via::
 
     $ make ARCH=armv7m
 
+Same applies for optional architecture flags via::
+
+    $ make ARCH=rv32imc ARCH_FLAGS=zba
+
 Module usage in MicroPython
 ---------------------------