libdw: make dwarf_getalt and dwarf_setalt thread-safe

libdw: make dwarf_getalt and dwarf_setalt thread-safe

	* libdw/dwarf_begin_elf.c (dwarf_begin_elf): Init dwarf_lock.
	* libdw/dwarf_end.c (dwarf_end): Free dwarf_lock.
	* libdw/dwarf_getalt.c (dwarf_getalt): Add locking.
	* libdw/dwarf_setalt.c (dwarf_setalt): Ditto.
	* libdw/libdwP.h (struct Dwarf): Define dwarf_lock.

Signed-off-by: Heather S. McIntyre <hsm2@rice.edu>
Signed-off-by: Aaron Merey <amerey@redhat.com>
Signed-off-by: Mark Wielaard <mark@klomp.org>