Remove handling_trap variable (#4963)

This historically was used to guard against recursive faults but
later refactorings have made this variable somewhat obsolete. The code
that it still protects is not the "meat" of trap handling. Instead the
`jmp_buf_if_trap` is changed to be more like "take" so once a "take"
succeeds it won't be able to recursively call any more "meat".

Overall this shouldn't affect anything, it's just a small internal
cleanup.

crates/runtime/src/traphandlers/unix.rs

@@ -87,7 +87,7 @@ unsafe extern "C" fn trap_handler(
         // handling, and reset our trap handling flag. Then we figure
         // out what to do based on the result of the trap handling.
         let (pc, fp) = get_pc_and_fp(context, signum);
-        let jmp_buf = info.jmp_buf_if_trap(pc, |handler| handler(signum, siginfo, context));
+        let jmp_buf = info.take_jmp_buf_if_trap(pc, |handler| handler(signum, siginfo, context));
 
         // Figure out what to do based on the result of this handling of
         // the trap. Note that our sentinel value of 1 means that the