improve abort identification in KLEE output

KLEE reports the line that called `ll::abort()`. The previous
solution called a safe `abort()` which in turn called `ll::abort`. This
meant that every abort was reported as originating in this library.

This replaces the safe `abort()` function with a macro `kabort!()`. The
behavior differs in that `kabort!()` expands to nothing when not in KLEE
mode, while `abort()` expanded to a panic.

The macro causes the call to `ll::abort()` to happen directly from user
code from KLEE's point of view.