Bug 1728549 - Remove some platform dependencies in WasmBCMemory. r=jseward

Remove platform dependencies that depend only on whether the platform
has a HeapReg or not, by introducing a properly named #define for this
and rewriting the code slightly.

Introduce a RegIntptr alias to express integer-representing-pointer
appropriately on different platforms.

Introduce a templated move() method for register-to-register moves.

Drive-by cleanup:

Replace the overloaded free() methods for freeing registers with a
templated variant to match what we have for push, pop, need, and move.

Document the internal #defines that control the compiler.

Differential Revision: https://phabricator.services.mozilla.com/D124642