[flang] Support extention intrinsic ABORT

The semantic checks and runtime have been supported. This supports the
lowering of intrinsic ABORT.

`gfortran` prints a backtrace before abort, unless `-fno-backtrace` is
given. This is good to use. The intrinsic BACKTRACE is not supported
yet, so add TODO in the runtime.

This extention is needed in SPEC2017 521.wrf_r in
https://github.com/llvm/llvm-project/issues/55955.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D130439