Fetch blackbox primitives from ISE for Spartan 6.

[mwkmwkmwk]

Steps to reproduce the issue

Provide instructions for reproducing the issue. Make sure to include
all necessary source files. (You can simply drag&drop a .zip file into
the issue editor.)

Also, make sure that the issue is actually reproducable in current git
master of Yosys.

See https://stackoverflow.com/help/mcve for some information on how to
create a Minimal, Complete, and Verifiable example (MCVE).

Please do not waste our time with issues that lack sufficient information
to reproduce the issue easily. We will simply close those issues.

Contact https://www.symbioticeda.com/ if you need commercial support for Yosys.

Expected behavior

Please describe the behavior you would have expected from the tool.

Actual behavior

Please describe how the behavior you see differs from the expected behavior.