Auto merge of #50369 - pftbest:unicode, r=SimonSapin
Auto merge of #50369 - pftbest:unicode, r=SimonSapin

Fix a warning in libcore on 16bit targets.

This code is assuming that usize >= 32bits, but it is not the case on
16bit targets. It is producing a warning that can fail the compilation
on MSP430 if deny(warnings) is enabled.
It is very unlikely that someone would actually use this code on
a microcontroller, but since unicode was merged into libcore we
have to compile it on 16bit targets.

I've tried to make sure that the code stays the same on x86,
here is an assembly comparison: https://godbolt.org/g/wFw7dZ

r? @SimonSapin

GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust