Gets rid of:
  avrbinutils
  avrgcc

to replace with:
  pkgsCross.avr.buildPackages.binutils
  pkgsCross.avr.buildPackages.gcc

- respect libc’s incdir and libdir
- make non-unix systems single threaded
- set LIMITS_H_TEST to false for avr
- misc updates to support new libc’s
- use multilib with avr

For threads we want to use:
- posix on unix systems
- win32 on windows
- single on everything else

For avr:
- add library directories for avrlibc
- to disable relro and bind
- avr5 should have precedence over avr3 - otherwise gcc uses the wrong one

Fixes #47533

newlib seems to think it’s a compiler when it’s not. Anyway had to
make host refer to build platform for things to work.

There are some weird ones out there that don’t follow any pattern:

- arm-none-eabi
- powerpc-none-eabi
- aarch64-none-elf

adds:

- arm-embedded
- avr
- aarch64-embedded
- powerpc-embedded

These two are unmaintained, no reason to keep them when they are
out-of-date

These packages should in theory work with our GCC toolchains, but
there are some definite breakages that need to be tracked down.
Comparing output of these to old gcc-arm-embedded is important.

Affected packages include:

- axolooti
- avrdudess
- opentx
- microscheme
- betaflight
- inav
- blackmagic
- simavr
- gnuk

replace with:

- pkgsCross.arm-embedded.buildPackages.gcc
- pkgsCross.arm-embedded.buildPackages.binutils

mentorToolchains.armLinuxGnuEabi is replaced with:

- pkgsCross.pogoplug4.buildPackages.gcc
- pkgsCross.pogoplug4.buildPackages.binutils

and mentorToolchains.armEabi is replaced with:

- pkgsCross.arm-embedded.buildPackages.gcc
- pkgsCross.arm-embedded.buildPackages.binutils

Fixes #39087

A convenient way to test embedded projects

Fixes #28160

avr: use new cross compilation infrastructure