s390/bpf: Implement bpf_arch_text_poke()

bpf_arch_text_poke() is used to hotpatch eBPF programs and trampolines.
s390x has a very strict hotpatching restriction: the only thing that is
allowed to be hotpatched is conditional branch mask.

Take the same approach as commit de5012b4

 ("s390/ftrace: implement
hotpatching"): create a conditional jump to a "plt", which loads the
target address from memory and jumps to it; then first patch this
address, and then the mask.

Trampolines (introduced in the next patch) respect the ftrace calling
convention: the return address is in %r0, and %r1 is clobbered. With
that in mind, bpf_arch_text_poke() does not differentiate between jumps
and calls.

However, there is a simple optimization for jumps (for the epilogue_ip
case): if a jump already points to the destination, then there is no
"plt" and we can just flip the mask.

For simplicity, the "plt" template is defined in assembly, and its size
is used to define C arrays. There doesn't seem to be a way to convey
this size to C as a constant, so it's hardcoded and double-checked
during runtime.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230129190501.1624747-4-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>