install rts/libidris_rts.a in libdir not datadir

[juhp]

(edit: rewritten to focus on libidris_rts.a)

Idris should install libidris_rts.a in libdir not datadir.

(I realised that .ibc files seem to be zip-archives. Are they arch (endian) independent?)

[jfdm]

i suppose we should reevaluate, at somepoint, how Idris handles and stores: packages; rts; codegens; and documentation. Not to mention how we use environment variables and options.

[juhp]

To answer my own question it seems .ibc files are arch independent (okay I only checked x86_64 vs i686).