RISC-V: NFC: Move scalar block move expansion code into riscv-string.cc

This just moves a few functions out of riscv.cc into riscv-string.cc in an
attempt to keep riscv.cc manageable.  This was originally Christoph's code and
I'm just pushing it on his behalf.

Full disclosure: I built rv64gc after changing to verify everything still
builds.  Given it was just lifting code from one place to another, I didn't run
the testsuite.

gcc/
	* config/riscv/riscv-protos.h (emit_block_move): Remove redundant
	prototype.  Improve comment.
	* config/riscv/riscv.cc (riscv_block_move_straight): Move from riscv.cc
	into riscv-string.cc.
	(riscv_adjust_block_mem, riscv_block_move_loop): Likewise.
	(riscv_expand_block_move): Likewise.
	* config/riscv/riscv-string.cc (riscv_block_move_straight): Add moved
	function.
	(riscv_adjust_block_mem, riscv_block_move_loop): Likewise.
	(riscv_expand_block_move): Likewise.