add and use CMake modules for valgrind client requests and quadruple precision math

this adds cmake modules for __float128 (quadruple precision) math which is available on some newish GCCs and valgrind client requests. so far, they are not yet used by anything, but I hope to uncloak a new module for discussion next week which has code to support these features. Since opm-core is the central hub for the build system, I thought that I  post them there in advance...