Update PRU assembler to corect hardware register numbering for DWARF.

Update PRU assembler to corect hardware register numbering for DWARF.

A small rework of the PRU GCC port exposed that CIE data alignment is
erroneously set to 4 for PRU in GAS. In fact PRU stack must be aligned to 1.
Set the macro to -1, to allow output from GCC to be assembled without errors.

Also, while at it, set DWARF2 HW register numbering to follow latest

	* config/tc-pru.c (pru_regname_to_dw2regnum): Return the starting HW
	byte-register number.
	(pru_frame_initial_instructions): Use byte-numbering for FP index.
	* config/tc-pru.h (DWARF2_DEFAULT_RETURN_COLUMN): Use number from
	latest GCC.
	(DWARF2_CIE_DATA_ALIGNMENT): Set to -1.