PPC: undefweak dynamic relocs

This makes the default for ppc to keep dynamic relocs on undefweak
symbols when the code won't cause DT_TEXTREL (for instance when -fPIE
or -fPIC).  If ld is given -z dynamic-undefined-weak then dynamic
relocations will be created for non-PIC at the expense of possibly
causing DT_TEXTREL to be set on ppc32.  Note that DT_TEXTREL and GNU
indirect functions are incompatible.

	* elf32-ppc.c (allocate_dynrelocs): Keep dyn_relocs for undefweak
	symbols when -z dynamic-undefined-weak or when there are no
	dynamic relocs in read-only sections and -z nodynamic-undefined-weak
	is not given.
	* elf64-ppc.c (allocate_dynrelocs): Likewise.