Add preferred base to DR and drmodtrack module data

[derekbruening]

For some use cases, the preferred base of a module is useful. In our case, our internal symbolization uses the binutils addr2line convention of two different types of PC: either an absolute PC for a non-relocatable executable (i.e., with a non-zero preferred==required base), or an offset from the load base for a position-independent library or executable (which has a zero preferred base for ELF; this design does not seem very cross-platform though as other types of binaries have non-zero preferred bases yet are relocatable). For this dual PC interface, there is not enough information in the module data recorded during tracing. This is a feature request to augment the trace module data (which will also require augmenting DR's public module interface) with either the preferred base or a flag indicating whether the module was at its preferred base.