Unbreak build on OpenBSD by not adding -Wl,-z,defs to linker flags.

This is similar to the fix for FreeBSD in r226862. Without this patch,
the build aborts when linkling libLTO.so, complaining about undefined
references to assert2, cxa_atexit, etc.

Patch by Stefan Kempf!

http://reviews.llvm.org/D14236

llvm-svn: 253769