Add UPC link support for FreeBSD

This is sufficient to compile and link a UPC "Hello, World!" program
with clang-upc on FreeBSD-10.0-RELEASE-p4/amd64.  However, the runtime
fails at startup:

$ ./a.out -n1
./a.out: UPC initialization failed.
./a.out: reason: Invalid argument
Abort trap (core dumped)

Additionally, I needed to pass -DLIBUPC_ENABLE_BACKTRACE=FALSE to
cmake.  While backtrace() and backtrace_symbols() are present in
FreeBSD-10.0 (though missing in earlier releases), are in a separate
libexecinfo.  Some additional logic could/should be added to link with
"-lexecinfo" when backtrace support is enabled on FreeBSD.
Add UPC link support for OpenBSD

This is sufficient to compile and link a UPC "Hello, World!" program
with clang-upc on OpenBSD-5.5/amd64.
However, the runtime fails at startup:

$ ./a.out -n1
./a.out: UPC initialization failed.
./a.out: reason: Invalid argument
Abort trap (core dumped)

Additionally, I needed to pass -DLIBUPC_ENABLE_BACKTRACE=FALSE to
cmake, because OpenBSD lacks backtrace() and backtrace_symbols().