xtensa: Fix inaccuracy in xtensa_rtx_costs()

xtensa: Fix inaccuracy in xtensa_rtx_costs()

This patch fixes the following defects in the function:

   - The cost of move instructions larger than the natural word width,
     specifically "movd[if]_internal", cannot be estimated correctly
   - Floating-point or symbolic constant assignment insns cannot be
     identified as L32R instructions

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_is_insn_L32R_p):
	Rewrite to capture insns that could be L32R machine instructions
	wherever possible.
	(xtensa_rtx_costs): Fix to consider that moves larger than a
	natural word can take multiple L32R machine instructions.
	(constantpool_address_p): Cosmetics.
	* config/xtensa/xtensa.md (movdi_internal, movdf_internal):
	Add missing insn attributes.