selftests/bpf: Add test for global data of a program in Rust

data_in_arena_rust.rs shows why libbpf keeps .data, .bss and .rodata of
a program in Rust in arena. A reference in Rust is an address that can
be stored in data. The list in the test has a node in each of the three
sections. IN_BSS.next is stored by the program, IN_DATA.next is
a relocation in .data against .rodata. sum() follows them and reads all
nodes with the same insns:

  5: w0 = *(u32 *)(r1 + 0x8)
  8: r1 = *(u64 *)(r1 + 0x0)

The same program without .arena.data section, when the sections are
array maps:

  libbpf: elf: skipping relo section(9) .rel.data for section(8) .data
  ...
  16: (79) r1 = *(u64 *)(r1 +0)         ; frame1: R1=scalar()
  ...
  13: (61) r0 = *(u32 *)(r1 +8)
  R1 invalid mem access 'scalar'

The program is built by upstream rustc for its bpfel-unknown-none
target. Nothing is done to the output: rustc emits LLVM bitcode and
clang makes the object of it for the cpu version of the test_progs
flavor. rustc has no prebuilt core for the target, so core is built from
the source that comes with rustc, like the kernel does it. That takes
5 seconds.

The subtest is skipped when:
- there is no rustc or it is older than 1.87. core is built as edition
  2024, which it is since then.
- rustc comes without the source of core (rustup component add rust-src).
- clang is older than 23 or older than LLVM of rustc. It can't read
  the bitcode then.

There are no kfuncs in the program. rustc doesn't emit debug info for
extern functions, so there would be no BTF for them.

panic=abort is a stop gap. Nothing in the program panics, so the panic
handler is not loaded. The series from Yonghong are about to add support
for panic=unwind.

Tested with rustc 1.95.0-nightly (LLVM 22.1) and clang 24.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20261002124714.180012-16-alexei.starovoitov@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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