Fix N^2 behavior in _bfd_dwarf2_find_symbol_bias

Fix N^2 behavior in _bfd_dwarf2_find_symbol_bias

A customer reported a case where addr2line was very slow.  We tracked
this down to some N^2 behavior in _bfd_dwarf2_find_symbol_bias in the
unusual case where no function can be found.

This patch fixes the bug, and reduces the runtime for a particular
request from 127 seconds to 1 second.

bfd/ChangeLog
2019-08-19  Tom Tromey  <tromey@adacore.com>

	* dwarf2.c (_bfd_dwarf2_find_symbol_bias): Create hash table
	holding symbols.