build: portability fixes ...

- Invoke Python scripts with `python3` if `python` is not available.

  Despite SciresM preferring Python 2, the scripts used in the build all
  worked in Python 3 already, so this is just a Makefile change.

- Export `MAKE` to work around a GNU bug.

  See the comment for details.

- `rmdir --ignore-fail-on-non-empty` -> `rmdir 2>/dev/null`

  macOS `rmdir` doesn't support `--ignore-fail-on-non-empty`.

  This is a slight downgrade in functionality, because now we ignore all
  errors from `rmdir` rather than just 'Directory not empty'.  I could
  have avoided this with a more complicated fix, but the benefit strikes
  me as not worth the complexity.  Let me know if you disagree.

- Append `$(DEVKITPRO)/tools/bin/` to `$PATH` when invoking
  `build_romfs`.

  This avoids the need to add devkitPro directories to `$PATH` when
  building Atmosphere.

  If you already have `build_romfs` in your PATH, then that will still
  take precedence.  (This seemed like the nicer option, though I don't
  have strong opinions.)

exo: use #embed for loader stub

ams: basic support for compiling with gcc 15

fusee/exo/ams: update with new keydata/version enums

fusee/exo/ams: update with new keydata/version enums

fusee/exo/ams: update with new keydata/version enums

bpc.mitm/exo: support pmic reboot/shutdown on mariko (thanks @CTCaer)

exo: fix up new titlekey option extents

exo/spl: Add new EsCommonKeyType

fusee/exo: implement the usual changes for new firmware support

fusee/exo: implement the usual changes for new firmware support

fusee/exo: fix up dram id names/mappings

fusee/exo: enum support for 15.0.0

ams: globally prefer R_RETURN to return for ams::Result

ams: use R_SUCCEED, R_THROW globally

exo: fix typo causing mishandling of carveouts

exo: right, Mariko* is a wiki-ism

exo: update for new dram ids

exo/fusee: first pass at updating enums for 14.0.0

abort/error: print backtrace, abuse templates, overhaul result/diag (macos not done yet)

ams: support building unit test programs on windows/linux/macos

util: better match true std::atomic semantics

exo: use the fancy new optimal atomics

ams: allow bootloader to merely approximate correct target firmware

spl: refactor for accuracy/move into libstrat

exo/vapours: refactor member variables to m_ over this->

ams: replace most remaining operator & with std::addressof

settings: implement KeyValueStore (#1659) ...

* settings: implement KeyValueStore and dependencies

* settings: update KeyValueStore for recent refactoring

* settings: address feedback

ams: the copyright^H^H^H^H^H^H^Hmplex plane is the algebraic closure of the reals

ams-libs: AMS_ASSERT no longer invokes expression