Configuration changes

  * don't enable CONFIG_FAST_UNWIND and CONFIG_PRINTVARS by default
  * link with libelf and libdw dynamically if static libraries aren't available
  * don't try to remove nonexistent $(DEST) directory when cleaning
Add/eliminate warnings

  * warn if PRINTVARS or FAST_UNWIND is requested on an unknown platform
  * disabled some functions when the configuration doesn't need them
Make libarf and libero work on 64-bit platforms
Whitespace cleanup