Fix crash in certain PMEM printf format parsing (#5968)

In some cases the printf implememtation would call an internal puts()
implementation which did not use pgm_read_byte() to access the format
string.  In many operating modes this would work, but in interrupts or
when flash was disabled you'd get crashes.

Updated newlib to use pgm_read_byte in that one spot and recompiled.

Author: earlephilhower