These were previously in the panic_unwind crate with dummy stubs in the
panic_abort crate. However it turns out that this is insufficient: we
still need a proper personality function even with -C panic=abort to
handle the following cases:

1) `extern "C-unwind"` still needs to catch foreign exceptions with -C
panic=abort to turn them into aborts. This requires landing pads and a
personality function.

2) ARM EHABI uses the personality function when creating backtraces.
The dummy personality function in panic_abort was causing backtrace
generation to get stuck in a loop since the personality function is
responsible for advancing the unwind state to the next frame.

The indirection is no longer needed since we always link to libgcc
even when the panic_abort runtime is used. Instead we can just call
the libgcc functions directly.