Build tor with -ftrapv by default
If the CFLAGS passed to tor don't contain -ftrapv (or the undefined behaviour sanitiser), let's build with -ftrapv.
This resolves #13538 (moved) non-intrusively, and prevents an entire class of integer safety bugs. (That said, wrapping can hide some issues rather than resolving them.)
To upload designs, you'll need to enable LFS. More information