Unexpected behaviour on x86-64 when building SDK using LLVM

This issue does not affect any released versions of Microkit since the SDKs available for download are using GCC.

This also does not seem to affect anyone using LLVM for their program images (at TS that is what we do by default for our projects using Microkit), we've only had issues when the SDK itself is built with LLVM.

More investigation required.