Add option --weak-unresolved-symbols to treat unresolved as weak references.

Add option --weak-unresolved-symbols to treat unresolved as weak references.

This patch adds option --weak-unresolved-symbols to treat unresolved symbols as
weak references.  This is helpful when we want the link to succeed with unresolved
symbols and the dynamic loader to not complain at run-time.  Option
--warn-unresolved-symbols lets the link succeed but could fail at run-time with
unresolved symbol warnings especially when the unresolved symbols have GOT entries
and dynamic relocations against them, like when -fPIE is used.

2015-04-23  Sriraman Tallam  <tmsriram@google.com>
	* options.h (--weak-unresolved-symbols): New option.
	* symtab.cc (Symbol_table::sized_write_globals): Change symbol
	binding to weak with new option.
	* symtab.h (is_weak_undefined): Check for new option.
	(is_strong_undefined): Check for new option.
	* testsuite/Makefile.am (weak_unresolved_symbols_test): New test.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/weak_unresolved_symbols_test.cc: New file.