selftests/bpf: add test for LDX/STX/ST relocations over array field

Add a simple repro for the issue of miscalculating LDX/STX/ST CO-RE
relocation size adjustment when the CO-RE relocation target type is an
ARRAY.

We need to make sure that compiler generates LDX/STX/ST instruction with
CO-RE relocation against entire ARRAY type, not ARRAY's element. With
the code pattern in selftest, we get this:

      59:       61 71 00 00 00 00 00 00 w1 = *(u32 *)(r7 + 0x0)
                00000000000001d8:  CO-RE <byte_off> [5] struct core_reloc_arrays::a (0:0)

Where offset of `int a[5]` is embedded (through CO-RE relocation) into memory
load instruction itself.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20250207014809.1573841-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>