tools/mpy_ld.py: Add new mpy_ld.py tool and associated build files.

This commit adds a new tool called mpy_ld.py which is essentially a linker
that builds .mpy files directly from .o files.  A new header file
(dynruntime.h) and makefile fragment (dynruntime.mk) are also included
which allow building .mpy files from C source code.  Such .mpy files can
then be dynamically imported as though they were a normal Python module,
even though they are implemented in C.

Converting .o files directly (rather than pre-linked .elf files) allows the
resulting .mpy to be more efficient because it has more control over the
relocations; for example it can skip PLT indirection.  Doing it this way
also allows supporting more architectures, such as Xtensa which has
specific needs for position-independent code and the GOT.

The tool supports targets of x86, x86-64, ARM Thumb and Xtensa (windowed
and non-windowed).  BSS, text and rodata sections are supported, with
relocations to all internal sections and symbols, as well as relocations to
some external symbols (defined by dynruntime.h), and linking of qstrs.

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