[cmake] Move LLVM_BUILD_STATIC check to an earlier point

Summary:
The motivation for this is to enable correct detection of dlopen() on Android.
Android does not provide a static version of libdl, so if we add the -static flag
after performing the check, it will succeed even though subsequent link steps
will fail. With this change we correctly detect the absence of libdl in a
LLVM_BUILD_STATIC build on Android.

The link itself still does not succeed because the code does not check the result
of this check properly, but I plan to fix that in a separate change.

Reviewers: beanz

Subscribers: danalbert, mgorny, srhines, tberghammer, llvm-commits

Differential Revision: https://reviews.llvm.org/D26463

llvm-svn: 287220