gas: make deferred expression evaluation generally latch dot

gas: make deferred expression evaluation generally latch dot

Deferring expression evaluation is often necessary. However, the value
current_location() records typically is intended to represent the
location at the point of use of the expression, with the exception being
.eqv (or its == equivalent). Change how expr_defer behaves in this
regard, and introduce a special mode just for pseudo_set() to use.

Introduce a predicate to cover both "deferred" modes, and use it
everywhere except in current_location(), where only the new mode wants
checking for.