backends: Add aarch64 native and core unwind support.

Add aarch64 backend functions frame_nregs and set_initial_registers_tid.
Mark pc_register in aarch64 prstatus_regs as pc_register.
Add backtrace-core-aarch64 testcase.

Signed-off-by: Mark Wielaard <mjw@redhat.com>
readelf: convert up to a target long in buf_read_ulong.

Add a i686 corefile test.

Reported-by: Markus Engel <m_engel@cs.uni-kl.de>
Signed-off-by: Mark Wielaard <mjw@redhat.com>
readelf: Read the decompressed data when processing .zdebug DWARF sections.

readelf uses libdw to open the Dwarf and read some of the DWARF data.
But it also uses its own parsers to display some of the low-level
unprocessed data. If the DWARF debug section was zlib compressed it
should actually use the decompressed section data from libdw instead
of the raw section data.

Includes a testcase for those sections that couldn't be properly
displayed when compressed before.

Signed-off-by: Mark Wielaard <mjw@redhat.com>