[Picolibc] Version 1.5.1

[Keith Packard]

Debian's autopkgtest caught a regression with the adoption of the
assembly version of the risc-v crt0 code -- '_exit' wasn't labeled as
weak causing link failures unless _exit was available. I've uploaded a
version that fixes just  this issue:

### Picolibc version 1.5.1

 1. Make riscv crt0 '_exit' symbol 'weak' to allow linking without
    this function.

-- 
-keith
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