kernel - Implement retpoline for kernel

kernel - Implement retpoline for kernel

* Now that we have gcc-8 operational, we can turn on retpoline (software
  spectre protection against the return stack buffer).  Turn it on via
  -mindirect-branch=thunk-inline

* No discernable performance loss with a generic buildkernel test:

				Xeon e5-2620v4 x 2
                        time make -j 32 nativekernel (all tmpfs)
BEFORE 1717.427u 323.662s 2:28.49 1374.5%      9582+721k 200842+0io 4870pf+0w
BEFORE 1720.130u 338.635s 2:30.21 1370.5%      9555+720k 199720+0io 4804pf+0w
BEFORE 1722.395u 341.508s 2:30.71 1369.4%      9559+720k 199720+0io 4804pf+0w

AFTER  1720.271u 329.492s 2:28.27 1382.4%      9578+721k 200842+0io 4870pf+0w
AFTER  1736.268u 344.874s 2:30.90 1379.1%      9555+720k 199720+0io 4804pf+0w
AFTER  1726.056u 348.324s 2:31.14 1372.4%      9543+719k 199720+0io 4804pf+0w