cmd/internal/obj/riscv: update references to RISC-V specification

Update references to version 20240411 of the RISC-V specifications.
Reorder and regroup instructions to maintain ordering. Also be
consistent with formatting.

The instruction encodings table was seemingly missed in CL 616115.

Change-Id: I47b7c8538383ff3b0503ba59db570c3d4f0d5653
Reviewed-on: https://go-review.googlesource.com/c/go/+/631935
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Meng Zhuo <[email protected]>
Reviewed-by: Pengcheng Wang <[email protected]>

Author: 4a6f656c

src/cmd/asm/internal/asm/testdata/riscv64.s

@@ -363,6 +363,10 @@ start:
 	SLLIUW		$63, X17, X18			// 1b99f80b
 	SLLIUW		$1, X18, X19			// 9b191908
 
+	//
+	// "B" Extension for Bit Manipulation, Version 1.0.0
+	//
+
 	// 28.4.2: Basic Bit Manipulation (Zbb)
 	ANDN	X19, X20, X21				// b37a3a41 or 93caf9ffb37a5a01
 	ANDN	X19, X20				// 337a3a41 or 93cff9ff337afa01