Adds support for writing values to AVR system I/O registers.

	* elf32-avr.c: Handle R_AVR_PORT5 and R_AVR_PORT6.
	* reloc.c: Add BFD_RELOC_AVR_PORT5 and BFD_RELOC_AVR_PORT6.
	* bfd-in2.h: Regenerate.
	* libbfd.h: Regenerate.

	* avr.h: Add R_AVR_PORT5 and R_AVR_PORT6.

	* config/tc-avr.c (avr_operand): Permit referring to r26-r31 by
	name as [xyz][hl].  Permit using a symbol whoes name begins with
	`r' to refer to a register.
	Allow arbitrary expressions for the P and p operators.
	(md_apply_fix): Check the BFD_RELOC_AVR_PORT5 and
	BFD_RELOC_AVR_PORT6 relocations.
Merge commit '75f580857a7c65525a98a701c65d0814be04155f' into avr-mainline

Conflicts:
	bfd/bfd-in2.h
	bfd/elf32-avr.c
	bfd/libbfd.h
	bfd/reloc.c
	gas/config/tc-avr.c
	include/elf/avr.h