src/readelf.c: Add support for print_debug_* output buffering

src/readelf.c: Add support for print_debug_* output buffering

Safely handle stdout output during concurrent calls to print_debug_*
functions.

For any print_debug_* function and any function that could be called
from print_debug_* which also prints to stdout: add a FILE * argument
and replace all printf, puts, putchar with fprintf.  All printing
to stdout will now be written to this FILE instead.

The FILE * is an interface to a per-thread dynamically-sized buffer.
libthread.a manages the allocation, printing and deallocation of
these buffers.

Signed-off-by: Aaron Merey <amerey@redhat.com>