[cmake] Fix standalone+LLVM_LINK_LLVM_DYLIB builds (pr36687)

Summary:
To make this build work, I needed to add detection code for the pthread
library. This is necessary, because we have direct calls to these
libraries (instead of going through llvm) and in the standalone build we
cannot rely on llvm to detect these for us. In a standalone non-dylib
build this was accidentaly working because these libraries were pulled
in as an interface dependency of the .a files, but in a dylib build
these are no longer part of the link interface, and so we need to add
them explicitly.

Reviewers: krytarowski, zturner

Subscribers: lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D44379

llvm-svn: 327490