[machdeps] ppc_32.json is closer to original; new avr_16.json machdep

- ppc_32 was originally configured for a Mac; using
`powerpc-apple-darwin` triple in clang mimicks that better than
a generic `powerpc` (mostly, some alignment changes, and char gets signed)
- x86_16 cannot be created with standard gcc/clang (they do not support
16 bit, `-m16` doesn't do much in fact). avr_16 seems more standard for
these compiler. x86_16 could be recreated with more exotic compilers.