newlib: drop phoenix support

newlib: drop phoenix support

This code has not been updated since 2016, and it looks like it has
rotted quite a bit since.  It does not build against the current set
of phoenix sources -- I had to hack both the kernel headers and the
newlib headers up to get it to build, and I still have no idea if it
actually links or runs.  It seems like the project itself has moved
away from newlib and to its own C library:
https://phoenix-rtos.com/documentation/libc/README.md

So since there's no interest from the phoenix folks to maintain this,
and it has a significant amount of non-standard code that we try to
keep up-to-date (without actually testing it), just punt it all.