[CMake] Use PUBLIC link mode for static libraries

Using INTERFACE prevents the use of imported libraries as we've done
in 00b3d49d because these aren't linked against the target, they're
only made part of the interface. This doesn't affect the output since
static libraries aren't being linked into, but it enables the use of
imported libraries.

Differential Revision: https://reviews.llvm.org/D74106

(cherry picked from commit 50a6d3a6)