IAR generates warning Pe111 'statement is unreachable'. In a couple of
cases, replace return statements with TU_ATTR_FALLTHROUGH; because some
compilers apparently can't figure out that the return statements are
unreachable but do whinge about an imagined fall-through without them!
- remove "-Wno-stringop-overflow -Wno-array-bounds"
- skip -Wconversion for gcc 9 and prior
- suppress_tinyusb_warnings only when building with gcc 9 and below
* gcc 9.2.1 has some spurious -Wconversion warnings
* cmake 3.18 and above require set_target_properties to be added from the target directory (so added it to all examples)
* fixed a few warnings in a couple of examples