libicp_rescue: unbreak for armv6 after recent OpenZFS import

Similar to fb17dfa0c83c, fix libicp_rescue to include asm versions of
sha2 on armv6, to unbreak the build of rescue.

Fixes: 3494f7c019fc
(cherry picked from commit 1a4a9a50574d9b4b4db90a85bc253d340c93a8a0)