genTargetHelperCallFor provides framework for calling Subzero
runtime functions. This patch implements calls and tests for
some of the available functions, but not all. Current
lowerCall implementation does not allow more (Handling of FP
return values is missing).

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2317653004 .

Patch from Srdjan Obucina <srdjan.obucina@imgtec.com>.