At present, macro PRIu8 resolves to "hhu", which does not seem to be supported by newlib's printf() implementation.
The same holds valid for the other PRI*8 macros. _intsup.h defines macro __INT8 to "hh". I guess this is technically correct, as "hh" should be correct for types of width CHAR_BITS, but it breaks in conjunction with newlib.
A quick search has shown that this problem comes up on other platforms as well:
I realise newlib's printf() has a trimmed-down feature-set as opposed to the POSIX version and may not support "hh" length modifiers for that specific reason. In this case, I propose to have macro __INT8 resolve to "h" on the MSP430 platform. This may not be POSIX compliant, but neither is the intended recipient at present.