erasure-code: create an internal isa lib for testing

erasure-code: create an internal isa lib for testing

we were linking against a '--module' shared library using its
.la file, which is not portable on some platforms accroding to
autotools. so instead of linking against the plugin so, a dedicated
internal library "libisa.la" is created for testing. and it is added
to the libec_isa.so also.

Signed-off-by: Kefu Chai <kchai@redhat.com>