kern: RESERVED_2F now set by HandleFpuException

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.)

kern: update synchronous exception handlers to assume FAR validity on TLB conflict

kern: update crt0 to reflect 20.0.0 changes

kern: update KernelLdr for 19.0.0 (new checks, dummy function call). ...

Also, fix a few very embarassing mistakes in kernel ldr:
* We have been mapping the page table region RWX for a few years now, accidentally.
* My attempt at making initial page tables not use bit 58 was broken in multiple ways.

kern: invoke supervisor mode thread functions from C++ context with valid stack frame

kern: add note that N ifdef'd out calling HandleException() for EL1 faults

kern: move KTargetSystem into .rodata, split init/verify

kern: allocate all TTBR0 pages during init, use procidx as asid

kern/ldr: enable the use of relr for relocations

kern: pass kernel base from KernelLdr to Kernel

kern: update KMemoryRegionType values for new ids + SecureUnknown region

kern: add speculation barriers after eret

kern: remove unnecessary fields from InitArgs (0x80 -> 0x40)

kern: on second thought, move vectors back to end of text

kern/ldr: move crt0 into .rodata

kern: pass ini1 size from loader to kernel, remove slab memset from init0

mesosphere: remove nostartfiles from specs files (should only be passed to linker wrapper)

kern: minor cleanup (thanks @liamwhite)

kern: refactor init (kill identity map, merge cpu on logic)

kern: move SecureAppletMemory/KPageBuffer heap into the ResourceRegion

kern: increase stack region size when thread resource limit is increased

kern: implement K(Secure)SystemResource

KScheduler: fix documentation mistakes

kern: turns out changing base address gives pretty elfs but breaks relocs

kern: codegen tweaks, now that we're no longer -Os

kern: update for new hw maintenance semantics

kern: implement new thread context/fpu semantics

kern: enforce end of dram == end of partitions (move our probably outdated KTraceBuffer to new location)

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