erasure-code: isa plugin must link with ErasureCode.cc

Otherwise it will not get the methods it needs. A test is added to check
the plugin loads as expected, from the command line. The test is not run
if the isa plugin is not found, which happens on platforms that are not
supported.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #2245 from dachary/wip-9085-isa-link

erasure-code: isa plugin must link with ErasureCode.cc

Reviewed-by: Sage Weil <sage@redhat.com>