[cmake] Rename append_if to avoid collision with LLVM

Rename the 'append_if' macro used in libunwind to 'unwind_append_if'.
Otherwise, when used in a combined LLVM+libunwind build, it overrides
the *incompatible* 'append_if' function from LLVM and breaks projects
following libunwind, e.g. OpenMP.

Differential Revision: https://reviews.llvm.org/D55476

llvm-svn: 348852