The larger alignment on this platform means that long double reports
a sizeof 16 bytes, but it's underlying size is really just the 10
bytes of `f80`

C doesn't give us a way to see the "underlying" size of a type, so
this has to be caught by hand or by monitoring runtime memory. Luckily,
x86 and x86-64 are the only platforms that seem to use a non-power-of-two
type like this.