riscv: lib: optimize strlen loop efficiency

Optimize the generic strlen implementation by using a pre-decrement
pointer. This reduces the loop body from 4 instructions to 3 and
eliminates the unconditional jump ('j').

Old loop (4 instructions, 2 branches):
  1: lbu t0, 0(t1); beqz t0, 2f; addi t1, t1, 1; j 1b

New loop (3 instructions, 1 branch):
  1: addi t1, t1, 1; lbu t0, 0(t1); bnez t0, 1b

This change improves execution efficiency and reduces branch pressure
for systems without the Zbb extension.

Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Link: https://patch.msgid.link/20251218032614.57356-1-jiangfeng@kylinos.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: flynnjiang

arch/riscv/lib/strlen.S

@@ -21,13 +21,11 @@ SYM_FUNC_START(strlen)
 	 * Clobbers:
 	 *   t0, t1
 	 */
-	mv	t1, a0
+	addi	t1, a0, -1
 1:
-	lbu	t0, 0(t1)
-	beqz	t0, 2f
 	addi	t1, t1, 1
-	j	1b
-2:
+	lbu	t0, 0(t1)
+	bnez	t0, 1b
 	sub	a0, t1, a0
 	ret