Make C10_MOBILE consistent with how feature macros are usually used (#17481)

Make C10_MOBILE consistent with how feature macros are usually used (#17481)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17481

Usually, feature macros are either defined or undefined and checked accordingly.
C10_MOBILE was a weird special case that was always defined but either defined to 1 or to 0.

This caused a lot of confusion for me when trying to disable something from mobile build and it also disabled it
from the server build (because I was using ifdef). Also, I found a place in the existing code base that made
that wrong assumption and used the macro wrongly, see https://fburl.com/y4icohts

Reviewed By: dzhulgakov

Differential Revision: D14214825

fbshipit-source-id: f3a155b6d43d334e8839e2b2e3c40ed2c773eab6