Intercept LLVM standard C library intrinsics

[jovanbulck]

Problem. When working for instance with strings or structs, the compiler might insert i.a., hidden memcpy operations. These function calls are currently not picked up by the Sancus LLVM pass, as they are represented as intrinsics at the IR level.

Proposed solution. Sancus LLVM pass should recognize these special intrinsics, and replace them with a call into an intra-SM C library function stub (e.g. __sm#name#_memcpy).