Patch up issues with GN builds (pthread / libz)

Summary:
Fixes up two small issues with the gn build.

 1 - Ensures that the correct ldflag `-pthread` is provided, not just linking the library.
 2 - Ensures that libraries are linked in the same group as the dependencies. This fixes a problem where system libraries (libc) are involved in a link-order dependency that's not being fulfilled.

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80591