Don't try to guess sys header conditions, and simply check if each signal is defined.

CC: lldb-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2394

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@197173 91177308-0d34-0410-b5e6-96231b3b80d8