[Picolibc] Version 1.7.1

[Keith Packard]

This version fixes a couple of incompatibilities with C++ applications
that crept into version 1.7. I've updated the release process to include
building crosstool-ng and testing that toolchain to make sure C++
applications work with each future release.

### Picolibc version 1.7.1

 1. Add __cxa_atexit implementation to 'picoexit' path as required by
    C++

 2. Fix lack of 'hh' support in integer-only tinystdio printf path.

 3. Fix tinystdio __file flag initialization for C++ apps

-- 
-keith
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