module/icp/asm-arm/sha2: fix compiling on armv5/6

The `adr` insn in neon kernel generates an compiling
error on armv5/6 target. Fix that by using `ldr`.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Closes #15557

module/icp/asm-arm/sha2/sha256-armv7.S

@@ -1849,7 +1849,11 @@ zfs_sha256_block_neon:
 	stmdb	sp!,{r4-r12,lr}
 
 	sub	r11,sp,#16*4+16
+#if __ARM_ARCH__ >=7
 	adr	r14,K256
+#else
+	ldr	r14,=K256
+#endif
 	bic	r11,r11,#15		@ align for 128-bit stores
 	mov	r12,sp
 	mov	sp,r11			@ alloca