ProcSyms should treat the executable like any other mapped file when

ProcSyms should treat the executable like any other mapped file when
symbolizing

As reported in #3487, when `/proc/PID/exe`'s symlink points to a
mountns-relative path from a different mountns than the tracing process,
we can fail to open it as we don't prepend `/proc/PID/root` .

A few potential solutions were discussed in that issue, we settled on
treating the main exe like any other map in `/proc/PID/maps`. Since it's
always the first map we can reuse existing code and get rid of
exe-specific helpers.