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