module/icp/asm-arm/sha2: enable non-SIMD asm kernels on armv5/6

My merged pull request #15557 fixes compilation of sha2 kernels on arm
v5/6. However, the compiler guards only allows sha256/512_armv7_impl to
be used when __ARM_ARCH > 6. This patch enables these ASM kernels on all
arm architectures. Some compiler guards are adjusted accordingly to
avoid the unnecessary compilation of SIMD (e.g., neon, armv8ce) kernels
on old architectures.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Closes #15623