[RISC-V] Restore inline expansion of block moves on RISC-V in some cases

[RISC-V] Restore inline expansion of block moves on RISC-V in some cases

Edwin's patch to add a --param for a size threshold on block moves
inadvertently disabled using inline block moves for cases where the count is
unknown.  This caused testsuite regressions (I don't remember which test, it
was ~6 weeks ago if not longer).  I'd hoped Edwin would see the new failures,
but I suspect he's buried by transition stuff with Rivos/Meta.

This patch restores prior behavior when the count is unknown and no --param was
specified.

Bootstrapped and regression tested on both the BPI and Pioneer systems and
regression tested on riscv{32,64}-elf as well.

Pushing to the trunk after pre-commit CI does its thing.

gcc/
	* config/riscv/riscv-string.cc (expand_block_move): Restore using
	inlined memcpy/memmove for unknown counts if the param hasn't been
	specified.
	(expand_vec_setmem): Similarly for memset.