[Snek] [keith-packard/snek] 1a488c: Support older picolibc (< 1.6)

[Keith Packard]

  Branch: refs/heads/picolibc-old
  Home:   https://github.com/keith-packard/snek
  Commit: 1a488c67d68a68a532189f25098157c887e3c755
      https://github.com/keith-packard/snek/commit/1a488c67d68a68a532189f25098157c887e3c755
  Author: Keith Packard <keithp at keithp.com>
  Date:   2022-12-10 (Sat, 10 Dec 2022)

  Changed paths:
    M chips/qemu/snek-qemu.c
    M chips/qemu/snek-qemu.defs
    M chips/qemu/snek-qemu.h
    M chips/samd21/ao-snek.h
    M chips/samd21/ao-stdio.c

  Log Message:
  -----------
  Support older picolibc (< 1.6)

Older versions of picolibc didn't include strfromf, strtof or the
--crt0 gcc command line option. Work around these by checking versions
during the build and providing implementations of the str functions that
work with snek.

Signed-off-by: Keith Packard <keithp at keithp.com>