[libc++] Systematically define a macro to check whether a feature is unavailable

Previously, we inconsistently defined whether a feature was unavailable
on a given deployment target. We would have availability attributes for
all features, but only some features had the equivalent of a _HAS_NO_FOO
macro in the form of the DISABLE_FTM___foo macros. Instead, systematically
define a _HAS_NO_FOO macro, which makes it easier to understand how to
add availability markup for a new platform.

Differential Revision: https://reviews.llvm.org/D147226