For bare metal ARM & xtensa targets, passing -g will make the ELF file
larger but doesn't change the binary size. However, this means tools like
gdb, addr2line, etc can extract source-level information from the ELF.
Also standardise -ggdb to -g, these produce the exact same ELF file on
arm-none-eabi-gcc and will use DWARF format for all these ports.
This separates extmod source files from `py.mk`. Previously, `py.mk`
assumed that every consumer of the py/ directory also wanted to include
extmod/. However, this is not the case. For example, building mpy-cross
uses py/ but doesn't need extmod/.
This commit moves all extmod-specific items from `py.mk` to `extmod.mk` and
explicitly includes `extmod.mk` in ports that use it.
Signed-off-by: David Lechner <david@pybricks.com>
