Add 'openmp' compiler property

Bug: http://b/70692399

If a module has 'openmp: true', add '-fopenmp' to CFlags during any
compile step and pass the libomp runtime to the linker during any link
step.

Test: Build samples in http://aosp/572924
Change-Id: Ic2a6410ec69aae548edaf582ee41659b0058561e