Add support for aarch64-apple-darwin

Fix Apple Silicon build (#214)

My previous Apple Silicon build went through, so I thought it already
worked, but it turns out it accidentally built an armv7 build instead.
This actually fixes the Apple Silicon build. One thing to note in
particular is that on Apple Silicong `long double` is the same as
`double` while on Linux `long double` is a 128 bit double-double
format.

Co-authored-by: Elliot Saba <[email protected]>