[Picolibc] Version 1.7.2

[Keith Packard]

A couple of C++ compatibility fixes; with this, I have packaged and used
libstdc++ for debian. Patches for that are coming.

### Picolibc version 1.7.2

 1. Fix picolibc.ld to split C++ exceptions back apart (thanks to
    Khalil Estell)

 2. Add vsscanf to tinystdio (required for libstdc++).

 3. Also stick -isystem in C++ compile command to try and get
    picolibc headers to be used instead of newlib.

-- 
-keith
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