kernel: Add -mindirect-branch-cs-prefix for WITH_KERNEL_RETPOLINE

[emaste]

Summary

-mindirect-branch-cs-prefix adds a CS segment-override prefix (0x2e)
to each call or jmp to a retpoline thunk.  The CPU ignores the prefix,
but it extends the instruction to six bytes instead of five.  This
leaves enough room for a future kernel change to patch all of the call
sites at boot time to disable retpoline if not required.

Support for this option arrived in GCC 12 and Clang/LLVM 16.