[cmake] Link socket/nsl on SunOS in llvm-jitlink

llvm-jitlink and llvm-jitlink-executor make use of APIs that are
part of the socket and nsl libraries on SunOS systems (Solaris and
Illumos). Make sure they get linked.

Ran into this in Rust CI when cross-compiling LLVM 12 to these
targets.

Differential Revision: https://reviews.llvm.org/D97633