Teach compiler that ereport(>=ERROR) does not return

When elevel >= ERROR, we add an abort() call to the ereport() macro to
give the compiler a hint that the ereport() expansion will not return,
but the abort() isn't actually reached because the longjmp happens in
errfinish().

Because the effect of ereport() varies with the elevel, we cannot use
standard compiler attributes such as noreturn for this.