[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.

gcc/config/riscv/riscv-string.cc

@@ -1249,7 +1249,7 @@ expand_block_move (rtx dst_in, rtx src_in, rtx length_in, bool movmem_p)
 	       && length > riscv_memcpy_size_threshold)
 	return false;
     }
-  else
+  else if (riscv_memmove_size_threshold != -1)
     return false;
 
   /* Inlining general memmove is a pessimisation: we can't avoid having to
@@ -1641,7 +1641,7 @@ expand_vec_setmem (rtx dst_in, rtx length_in, rtx fill_value_in)
 	  && length > riscv_memset_size_threshold)
 	return false;
     }
-  else
+  else if (riscv_memset_size_threshold != -1)
     return false;
 
   rtx dst_addr = copy_addr_to_reg (XEXP (dst_in, 0));