For various reasons, CMake's detection mechanism for backtrace() returns an absolute path /usr/lib/libexecinfo.so on FreeBSD and NetBSD.
Since tools/llvm-config/CMakeLists.txt only checks if system libraries start with -, this causes llvm-config --system-libs to produce the following incorrect output:
-lrt -l/usr/lib/libexecinfo.so -ltinfo -lpthread -lz -lm
Fix it by checking for both - and /, assuming the latter indicates an absolute path to a library.
This also fixes the Bindings/Go/go.test test case, since that always died with "unable to find library -l/usr/lib/libexecinfo.so".