build: add -fpic as cmake option
build: add -fpic as cmake option

When building shared libraries, position independent code has to be
created. To enable this, add a cmake option "-DFPIC=ON", which adds
-fpic to the g++ calls.

Furthermore, add a shared riss-coprocessor-lib-shared build target to
build such a library.

nmanthey/riss-solver,nmanthey/riss-solver,nmanthey/riss-solver