s390/bpf: Implement bpf_jit_supports_kfunc_call()

Implement calling kernel functions from eBPF. In general, the eBPF ABI
is fairly close to that of s390x, with one important difference: on
s390x callers should sign-extend signed arguments. Handle that by using
information returned by bpf_jit_find_kfunc_model().

Here is an example of how sign extensions works. Suppose we need to
call the following function from BPF:

    ; long noinline bpf_kfunc_call_test4(signed char a, short b, int c,
long d)
    0000000000936a78 <bpf_kfunc_call_test4>:
    936a78:       c0 04 00 00 00 00       jgnop bpf_kfunc_call_test4
    ;     return (long)a + (long)b + (long)c + d;
    936a7e:       b9 08 00 45             agr     %r4,%r5
    936a82:       b9 08 00 43             agr     %r4,%r3
    936a86:       b9 08 00 24             agr     %r2,%r4
    936a8a:       c0 f4 00 1e 3b 27       jg      <__s390_indirect_jump_r14>

As per the s390x ABI, bpf_kfunc_call_test4() has the right to assume
that a, b and c are sign-extended by the caller, which results in using
64-bit additions (agr) without any additional conversions. Without sign
extension we would have the following on the JITed code side:

    ; tmp = bpf_kfunc_call_test4(-3, -30, -200, -1000);
    ;        5:       b4 10 00 00 ff ff ff fd w1 = -3
    0x3ff7fdcdad4:       llilf   %r2,0xfffffffd
    ;        6:       b4 20 00 00 ff ff ff e2 w2 = -30
    0x3ff7fdcdada:       llilf   %r3,0xffffffe2
    ;        7:       b4 30 00 00 ff ff ff 38 w3 = -200
    0x3ff7fdcdae0:       llilf   %r4,0xffffff38
    ;       8:       b7 40 00 00 ff ff fc 18 r4 = -1000
    0x3ff7fdcdae6:       lgfi    %r5,-1000
    0x3ff7fdcdaec:       mvc     64(4,%r15),160(%r15)
    0x3ff7fdcdaf2:       lgrl    %r1,bpf_kfunc_call_test4@GOT
    0x3ff7fdcdaf8:       brasl   %r14,__s390_indirect_jump_r1

This first 3 llilfs are 32-bit loads, that need to be sign-extended
to 64 bits.

Note: at the moment bpf_jit_find_kfunc_model() does not seem to play
nicely with XDP metadata functions: add_kfunc_call() adds an "abstract"
bpf_*() version to kfunc_btf_tab, but then fixup_kfunc_call() puts the
concrete version into insn->imm, which bpf_jit_find_kfunc_model() cannot
find. But this seems to be a common code problem.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230129190501.1624747-7-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>