Makefile: Improve unisim library generation

Makefile: Improve unisim library generation

The rewrite of the Makefile to use "ghdl -c" somewhat broke building
the unisim library as ghdl doesn't yet support putting files in
separate libraries from a single command line invocation.

The workaround at the time was to put the entire project in "unisim"
which is ... weird and will break if we try to add another library
such as fmf.

This fixes it by generating the library separately using "ghdl -i"

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>