Review relationship between local and global timestamps

[tmplt]

Current implementation assumes that local and global timestamps are sourced from the same clock. This is not neccesarily the case, and is platform dependent: global timestamps are sourced from a "global timestamp clock" and local timestamps can either be sourced from the async TPIU clock or the system processor clock.

We'll likely need to add some configuration options on how to interpret timestamp packets.

[tmplt]

Following from tmplt/masters-thesis@70ce53f it may be apt to inquire ARM whether the clock relationship should be interpreted as device-specific.