[libc] Make internal assertion call abort

I notice that LSB actually say that assert failure needs to **abort** the program. **abort** has a better semantic that gdb captures it by default. We need to fix that. `abort` allows reentrancy (recursive assertion failure) but current pure-header-library style makes cyclic dependency unavoidable. This should be cleaned up later.

Start by locating libc's internal assertion implementation and tracing its current failure path. Compare that behavior with the LSB requirement and inspect existing assertion-related tests; done means assertion failure invokes abort while preserving the stated reentrancy considerations.

Written by the indexing model from the issue text.