Add %z and %x to printf backend (#5424)

* Add %z and %x to printf backend

%z is a C99 format used for size_t and was not included in any printf.
On the 8266 it's a no-op as size_t==int, so ignore it and things just
work.

%x lowercase support added back in (wasn't present in nano-printf).

* Update to toolchain built newlib, fix link error

Previous commit was a hand build and copy, this one used the full
toolchain and should not include atexit().

Author: earlephilhower