rustup: add core_str_ext feature
rustup: add core_str_ext feature

This fixes failure on stm32l1 platform

Signed-off-by: Paul Osborne <b6afffd6c1a00645fdd04b09edf63d034a949a39@gmail.com>

farcaller/zinc,phil-opp/zinc,phil-opp/zinc,posborne/zinc,richo/zinc,hackndev/zinc,posborne/zinc,ntwyman/zinc,farcaller/zinc,richo/zinc,posborne/zinc,richo/zinc,hackndev/zinc,mcoffin/zinc,phil-opp/zinc,ntwyman/zinc,troplin/zinc,posborne/zinc,phil-opp/zinc,mcoffin/zinc,farcaller/zinc,mcoffin/zinc,ntwyman/zinc,richo/zinc,mcoffin/zinc,richo/zinc,mcoffin/zinc,troplin/zinc,hackndev/zinc,hackndev/zinc,hackndev/zinc,troplin/zinc,troplin/zinc,posborne/zinc,phil-opp/zinc,troplin/zinc,farcaller/zinc,ntwyman/zinc,ntwyman/zinc,farcaller/zinc