Implement -z dynamic-undefined-weak

Implement -z dynamic-undefined-weak

-z nodynamic-undefined-weak is only implemented for x86.  (The sparc
backend has some support code but doesn't enable the option by
including ld/emulparams/dynamic_undefined_weak.sh, and since the
support looks like it may be broken I haven't enabled it.)  This patch
adds the complementary -z dynamic-undefined-weak, extends both options
to affect building of shared libraries as well as executables, and
adds support for the option on powerpc.

include/
	* bfdlink.h (struct bfd_link_info <dynamic_undefined_weak>):
	Revise comment.
bfd/
	* elflink.c (_bfd_elf_adjust_dynamic_symbol): Hide undefweak
	or make dynamic for info->dynamic_undefined_weak 0 and 1.
	* elf32-ppc.c:Formatting.
	(ensure_undefweak_dynamic): Don't make dynamic when
	info->dynamic_undefined_weak is zero.
	(allocate_dynrelocs): Discard undefweak dyn_relocs for
	info->dynamic_undefined_weak.  Discard undef dyn_relocs when
	not default visibility.  Discard undef and undefweak
	dyn_relocs earlier.
	(ppc_elf_relocate_section): Adjust to suit.
	* elf64-ppc.c: Formatting.
	(ensure_undefweak_dynamic): Don't make dynamic when
	info->dynamic_undefined_weak is zero.
	(allocate_dynrelocs): Discard undefweak dyn_relocs for
	info->dynamic_undefined_weak.  Discard them earlier.
ld/
	* ld.texinfo (dynamic-undefined-weak): Document.
	(nodynamic-undefined-weak): Document that this option now can
	be used with shared libs.
	* emulparams/dynamic_undefined_weak.sh: Support -z
	dynamic-undefined-weak.
	* emulparams/elf32ppccommon.sh: Include dynamic_undefined_weak.sh.
	* testsuite/ld-undefined/weak-undef.exp (undef_weak_so),
	(undef_weak_exe): New.  Use them.  Add -z dynamic-undefined-weak
	and -z nodynamic-undefined-weak tests.
	* Makefile.am: Update powerpc dependencies.
	* Makefile.in: Regenerate.