* F80Repr extern struct needs no explicit padding; let's match the
   target padding.
 * stage2: fix lowering of f80 constants.
 * stage1: decide ABI size and alignment of f80 based on alignment of
   u64. x86 has alignof u64 equal to 4 but arm has it as 8.
 * stage2: fix Value.floatReadFromMemory to use F80Repr