Make DtoMutexType() independent of gIR.

This method may be called during semantic analysis. gIR is not initialized in this case.
Since only the LLVM context is required add a check and use the global LLVM context.
Remove magic constants from Target::critsecsize().

The type of a critical section is returned by DtoMutexType() and can be used to derive the size.
This is required for correct cross-compilation.