[Picolibc] recent tinystdio changes

[Keith Packard]

I made a couple of tinystdio changes over the last few days:

 * Used the linker instead of pre-processor to select between printf
   variants

   I finally figured out how to switch between printf variants using the
   linker by using --defsym. *Plus* I figured out how to make this work
   using the same command line option which defines a preprocessor
   symbol by matching on that in picolibc.specs.

 * Pulled the sprintf/sscanf support out of fgetc/fputc and into
   separate functions.

   String I/O now uses custom put/get functions. This saves a bunch of
   space in fgetc/fputc and streamlines that code in the usual case.

 * Made getc/ungetc re-entrant using atomic operations.

   For systems without the necessary atomic support, there are functions
   that applications can replace. This means that applications can use
   tinystdio from multiple threads without needing external locks, and
   without needing to add locking inside picolibc.

   The POSIX interfaces, which include buffering on top of POSIX I/O
   functions, are still non-reentrant. Those will require a locking
   interface to be provided by the underlying O/S.

On rv32imac, the size of the printf.c example is smaller now:

                        text before     text after
printf.elf              9284            8998
printf-float.elf        6540            6214
printf-int.elf          2336            2266

-- 
-keith
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