glasm: Ensure reg alloc order across compilers on GLASM

Use a struct constructor to serialize register allocation arguments to
ensure registers are allocated in the same order regardless of the
compiler used.

The A and B functions can be called in any order when passed as
arguments to "foo":

  foo(A(), B())

But the order is guaranteed for curly-braced constructor calls in
classes:

  Foo{A(), B()}

Use this to get consistent behavior.