[PECOFF] Use "llvm-readobj -coff-imports" to test the import table.

-coff-imports is added in r218915. You may have to sync your llvm source tree.
[Stackmaps] Make ithe frame-pointer required for stackmaps.

Do not eliminate the frame pointer if there is a stackmap or patchpoint in the
function. All stackmap references should be FP relative.

This fixes PR21107.