allegro: release-heap tweaks

Some architectures don't support :RELEASE-HEAP, so pass
:RELEASE-HEAP-IGNORABLE to FF:DEF-FOREIGN-CALL to avoid compile-time
warnings.

Also, be a bit more conservative and pass :WHEN-OK rather than :ALWAYS
in order to honor WITHOUT-INTERRUPTS. (See
<http://franz.com/support/documentation/9.0/doc/operators/ff/def-foreign-call.htm#note-3-release-heap>.)

Bug reported by Robert P. Goldamn and fix suggested by Anton Vodonosov
in <https://github.com/cl-plus-ssl/cl-plus-ssl/issues/13>.