[Picolibc] Picolibc version 1.4.2 released

[Keith Packard]

### Picolibc version 1.4.2

 1. Clang source compatibility. Clang should now be able to compile
    the library. Thanks to Denis Feklushkin for figuring out how
    to make this work.

 2. aarch64 support. This enables the existing aarch64 code and
    provides an example configuration file for getting it
    built.  Thanks for Anthony Anderson for this feature.

 3. Testing on github on push and pull-request. For now, this is
    limited to building the library due to a bug in qemu.

 4. Get newlib stdio working again. You can now usefully use Newlib's
    stdio. This requires a working malloc and is substantially larger
    than tinystdio, but has more accurate floating point input. This
    requires POSIX functions including read, write and a few others.

 5. Fix long double strtold for 128-bit types. The working version is
    only available when using tinystdio; if using newlib stdio, strtold
    is simply not available.

 6. Improve tinystdio support for C99 printf/scanf additions.

 7. Check for correct prefix when sysroot-install option is
    selected. The value of this option depends on how gcc was
    configured, and (alas) meson won't let us set it at runtime, so
    instead we complain if the wrong value was given and display the
    correct value.

 8. Sync up with current newlib master.

-- 
-keith
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