RISC-V: Change sp subtracts so prologue stores can compress.

RISC-V: Change sp subtracts so prologue stores can compress.

	gcc/
	* config/riscv/riscv.c (riscv_first_stack_step): Move locals after
	first SMALL_OPERAND check.  New local min_second_step.  Move assert
	to where locals are set.  Add TARGET_RVC support.
	* config/riscv/riscv.h (C_SxSP_BITS, SWSP_REACH, SDSP_REACH): New.

From-SVN: r257681