[Picolibc] Version 1.4.7

[Keith Packard]

We've managed to do quite a bit of picolibc over the last couple of
months! Here's version 1.4.7.

 1. Fix numerous libm exception and errno bugs. The math functions are
    all now verified to match the C19 and Posix standards in this
    area.

 2. Change behavior of 'gamma' function to match glibc which returns
    lgamma for this function. Applications should not use this
    function, they should pick either lgamma or tgamma as appropriate.
 
 3. Fix fma/fmaf on arm and RISC-V so that the machine-specific versions
    are used when the hardware has support. Also fix the math library
    to only use fma/fmaf when it is supported by the hardware.

 4. Fix numerous nano-malloc bugs, especially with unusual parameters.

 5. Change nano-malloc to always clear returned memory.

 6. Improve nano-realloc to perform better in various ways, including
    merging adjacent free blocks and expanding the heap.

 7. Add malloc tests, both a basic functional test and a stress test.

 8. Improve build portability to Windows. Picolibc should now build
    using mingw.

 9. Use hardware TLS register on ARM when available.

 10. Support clang compiler. Thanks to Denis Feklushkin
     <denis.feklushkin at gmail.com> and Joakim Nohlgård <joakim at nohlgard.se>.

 11. Avoid implicit float/double conversions. Check this by having
     clang builds use -Wdouble-promotion -Werror=double-promotion
     flags

 12. Have portable code check for machine-specific overrides by
     matching filenames. This avoids building libraries with
     duplicate symbols and retains compatibility with newlib (which
     uses a different mechanism for this effect).

 13. Patches to support building with [CompCert](http://compcert.inria.fr/), a
     formally verified compiler. Thanks to Sebastian Meyer
     <meyer at absint.com>.

-- 
-keith
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