src: move ABORT() logic into node::Abort()

Don't inline calls to node::DumpBacktrace() and fflush(), it makes the
generated code bigger.  A secondary benefit of moving it to a function
is that it gives you something to put a breakpoint on.

PR-URL: https://github.com/nodejs/node/pull/6734


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>