xtensa: Improve instruction cost estimation and suggestion

This patch implements a new target-specific relative RTL insn cost function
because of suboptimal cost estimation by default, and fixes several "length"
insn attributes (related to the cost estimation).

And also introduces a new machine-dependent option "-mextra-l32r-costs="
that tells implementation-specific InstRAM/ROM access penalty for L32R
instruction to the compiler (in clock-cycle units, 0 by default).

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_rtx_costs): Correct wrong case
	for ABS and NEG, add missing case for BSWAP and CLRSB, and
	double the costs for integer divisions using libfuncs if
	optimizing for speed, in order to take advantage of fast constant
	division by multiplication.
	(TARGET_INSN_COST): New macro definition.
	(xtensa_is_insn_L32R_p, xtensa_insn_cost): New functions for
	calculating relative costs of a RTL insns, for both of speed and
	size.
	* config/xtensa/xtensa.md (return, nop, trap): Correct values of
	the attribute "length" that depends on TARGET_DENSITY.
	(define_asm_attributes, blockage, frame_blockage): Add missing
	attributes.
	* config/xtensa/xtensa.opt (-mextra-l32r-costs=): New machine-
	dependent option, however, preparatory work for now.

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