[$] Relief for retpoline pain

Indirect function calls - calls to a function whose address is stored in apointer variable - have never been blindingly fast, but the Spectrehardware vulnerabilities have made things far worse. The indirect branchpredictor used to speed up indirect calls in the CPU can no longer beused, and performance has suffered accordingly. The "retpoline"mechanism was a brilliant hack that proved faster than the hardware-based solutionsthat were tried at the beginning. While retpolines took a lot of the painout of Spectre mitigation, experience over the last year has made it clearthat they still hurt. It is thus not surprising that developers have beenlooking for alternatives to retpolines; several of them have shown up onthe kernel lists recently.