New features

The SDK completes the first pass of porting cc65's stdio capabilities. It's not very well tested, so expect the unexpected. (I've given the commodore and atari ports some smoke testing, but not the rp6502. That is likely to work as-is though, since the cc65 POSIX stuff was already added to llvm-mos previously.)
- Notable exception: atari's lseek has not yet been ported, since it only works on Sparta DOS X. See #333
ELF POSIX utility script to generate atari800 label files (atari800lbl)

Bug fixes

#332 - Exit to Atari DOS via JMP (DOSVEC). The DUP clobber detection isn't robust in Atari DOS, so exiting via RTS may cause a crash. We could have fixed this by re-ordering the XEX file chunks, but reloading the DUP effects a JMP (DOSVEC) anyway, so may as well. - @cwedgwood
Add missing devnum symbol to non-c64 commodore targets' POSIX file I/O layers. This fixes stdio on these targets.