[libc++][NFC] Reformat features.py

This file was reformatted using the Black tool, which led to entirely
unreadable code due to how lines are broken. Formatting tools are fine,
but not when they lead to code that humans have trouble reading. In the
case of features.py, a lot of it was meant to be aligned in a repetitive
but consistent way to make the structure of the code stand out.
Reformatting with the tool lost that property.

Differential Revision: https://reviews.llvm.org/D152737