[drmemtrace] record branch target information for filtered traces

For non-filtered traces, branch target info can be inferred from the final trace by the tool. But for a trace with icache filtering this is not the case. To provide branch taken/not-taken and target information we'd have to insert an extra entry into the trace with the target, which for conditionals will require flag emulation logic, with corresponding additional overhead: so perhaps this should be controlled by a runtime flag.

Start by locating the drmemtrace implementation for icache filtering and the trace record format. Determine how filtered traces currently represent branches, then scope the proposed runtime-controlled target information and its conditional-branch flag handling. Done means the behavior, overhead control, and resulting branch target data are specified and validated.

Written by the indexing model from the issue text.