Enhance powerpc ld -r --relax

Enhance powerpc ld -r --relax

One of the ill effects of ld -r is to mash together sections.  That
can result in reduced icache performance at runtime due to unexpected
movement of code.  Another problem is that sections can become too
large to link on targets that have limited relative addressing.  ld -r
--relax attempts to overcome the large section problem for branches by
inserting trampolines, but the powerpc support added lots of
unnecessary trampolines.  This patch trims them somewhat.

bfd/
	* elf32-ppc.c (ppc_elf_relax_section): Ignore common or undef locals.
	Avoid trashing toff with added when used as a symbol index.
	Ignore R_PPC_PLTREL24 addends in unused example code.  Avoid
	creating unnecessary fixups when relocatable.
ld/
	* testsuite/ld-powerpc/big.s: New file.
	* testsuite/ld-powerpc/relaxrl.d: New test.
	* testsuite/ld-powerpc/powerpc.exp: Run new test.
	* testsuite/ld-powerpc/relaxr.d: Adjust.