Use binary search on dynamic relocations

Replace linear search with binary search on dynamic relocations.  After
finding a match, scan backward to the first matching relocation, then
scan forward for a matching relocation with non-absolute symbol.

On Fedora 27 x86-64, time for "objdump -d" on libxul.so from RHEL 7.4
x86-64 went from

134.46user 0.12system 2:15.03elapsed

to

8.49user 0.14system 0:08.64elapsed

	PR binutils/22911
	* objdump.c (is_significant_symbol_name): Return TRUE for all
	.plt* sections.
	(find_symbol_for_address): Replace linear search with binary
	search on dynamic relocations.