This fixes ICE on any source compiled with -mint8.
Missing definition of UINT16_TYPE (defined to 0) crashed
the compiler when it tries to build wchar stuff.

As mentioned in the PR, I chose to add the stdint stuff
as a new file.

The -mint8 part is only lightly tested because there is no
test suite for the non-C compliant code it generates.

Without -mint8 avr-stdint.h mimics as many as possible definitions
from newlib-stdint.h but for 2 cases where it deviates:

- SIG_ATOMIC_TYPE is 8-bit, AVR cannot access 16 bits atomically

- [U]INT_FAST8_TYPE is 8-bit because AVR is an 8-bit machine

Ok to install?

Johann

	PR target/46261
	* config/avr/avr-stdint.h: New file.
	* config.gcc (avr-*-*,tm_file): Use avr/avr-stdint.h instead of
	newlib-stdint.h