libicp: unbreak for armv6 after recent OpenZFS import

The following upstream commit:
727497ccdfcc module/icp/asm-arm/sha2: enable non-SIMD asm kernels on armv5/6

does indeed enable sha2 asm for earlier arm CPUs, but since libicp's
Makefile was not updated, this leads to:

ld: error: undefined reference due to --no-allow-shlib-undefined: zfs_sha256_block_armv7

Fix it by compiling sha256-armv7.S and sha512-armv7.S for armv6 too.

Fixes: 3494f7c019fc