bpf: Emit timed may_goto for JITed progs only

The timed form of may_goto calls arch_bpf_timed_may_goto(). This function
takes its argument and returns its result in BPF_REG_AX, in a register of
the cpu. Only JITed code can call it.

The verifier emits the timed form when the arch has a JIT that supports it.
It does not look at the prog. With CONFIG_BPF_JIT_ALWAYS_ON=n the
interpreter can run such a prog: the JIT is off, or it fails. The
interpreter calls the function like a helper. The function takes what the C
code left in that register for a pointer, and bpf_check_timed_may_goto()
reads and writes through it.

Emit the timed form only when the prog asks for a JIT, and make the JIT
mandatory for such a prog. All other progs get the form that counts down
from BPF_MAX_LOOPS.

One test of verifier_may_goto_1 shows the insns of the timed form. Load it
only when progs are JITed.

Fixes: e723608bf428 ("bpf: Add verifier support for timed may_goto")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261031215543.682284-2-alexei.starovoitov@gmail.com

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