Make cmake link flag naming consistent

Summary:
The build system was inconsistent in its naming conventions for
link flags. This patch changes all uses of LINKFLAGS to LINK_FLAGS,
for consistency with cmake's LINK_FLAGS property.

This patch should make it easier to search the source code for
uses of link flags, as well as providing the benefit of improved
style and consistency.

Reviewers: compnerd, beanz

Subscribers: kubabrecka, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28506

llvm-svn: 291539