[cmake] Add linker option "-Wl,-z,defs" in standalone build

Add the "-Wl,-z,defs" linker option that is used to prevent
underlinking. It is already used by LLVM itself but does not get
propagated into stand-alone build of libc++. This patch ensures
that the option is passed in independently of whether libc++ is built
in-tree or out-of-tree.

Patch by Lei Zhang.

Differential Revision: https://reviews.llvm.org/D24119

llvm-svn: 282483