ld: print branch fixups into the map file for ppc elf targets

In a safety context, it could interesting to track the trampolines being
generated, ensuring there are expected or not.

bfd/ChangeLog:

	* elf32-ppc.c (ppc_elf_relax_section): Log branch fixups.

ld/ChangeLog:

	* ld.texi (--print-map): Add new item about fixups.