Rewrite pic.md to improve medany and pic code size.

Rewrite pic.md to improve medany and pic code size.

	gcc/
	* config/riscv/pic.md: Rewrite.
	* config/riscv/riscv.c (riscv_address_insns): Return cost of 3 for
	invalid address.
	* config/riscv/riscv.md (ZERO_EXTEND_LOAD): Delete.
	(SOFTF, default_load, softload, softstore): New.

From-SVN: r263943