tools/mpy_ld.py: Allow linking static libraries.

This commit introduces an additional symbol resolution mechanism to the
natmod linking process.  This allows the build scripts to look for required
symbols into selected libraries that are provided by the compiler
installation (libgcc and libm at the moment).

For example, using soft-float code in natmods, whilst technically possible,
was not an easy process and required some additional work to pull it off.
With this addition all the manual (and error-prone) operations have been
automated and folded into `tools/mpy_ld.py`.

Both newlib and picolibc toolchains are supported, albeit the latter may
require a bit of extra configuration depending on the environment the build
process runs on.  Picolibc's soft-float functions aren't in libm - in fact
the shipped libm is nothing but a stub - but they are inside libc.  This is
usually not a problem as these changes cater for that configuration quirk,
but on certain compilers the include paths used to find libraries in may
not be updated to take Picolibc's library directory into account.  The bare
metal RISC-V compiler shipped with the CI OS image (GCC 10.2.0 on Ubuntu
22.04LTS) happens to exhibit this very problem.

To work around that for CI builds, the Picolibc libraries' path is
hardcoded in the Makefile directives used by the linker, but this can be
changed by setting the PICOLIBC_ROOT environment library when building
natmods.

Signed-off-by: Volodymyr Shymanskyy <vshymanskyi@gmail.com>
Co-authored-by: Alessandro Gatti <a.gatti@frob.it>

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