[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

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@348852 91177308-0d34-0410-b5e6-96231b3b80d8