Merge branch 'bpf, x64: optimize JIT's pro/epilogue'

Maciej Fijalkowski says:

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Hi!

This small set can be considered as a followup after recent addition of
support for tailcalls in bpf subprograms and is focused on optimizing
x64 JIT prologue and epilogue sections.

Turns out the popping tail call counter is not needed anymore and %rsp
handling when stack depth is 0 can be skipped.

For longer explanations, please see commit messages.

Thank you,
Maciej
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Signed-off-by: Alexei Starovoitov <ast@kernel.org>