selftests/bpf: map_kptr: Expect BPF_ST reject msg on cpuv4 toolchains

reject_scalar_store_to_kptr stores a scalar constant to a kptr field:

        *(volatile u64 *)&v->unref_ptr = 0xBADC0DE;

Compilers generate one of two encodings for that:

 1. Materialize the constant into a register and emit BPF_STX:

        r1 = 0xbadc0de
        *(u64 *)(r0 + 0x8) = r1

 2. Or fold it into a single BPF_ST (store immediate):

        *(u64 *)(r0 + 0x8) = 0xbadc0de

These go through different rejection paths and output different
messages.
 - BPF_STX goes through map_kptr_match_type(), which prints
   "invalid kptr access, R...".
 - BPF_ST only gets the immediate check printing
   "BPF_ST imm must be 0 when storing to kptr"

The test only expects the BPF_STX message, so it fails on a toolchain
that folds the constant - bpf-gcc, and clang -mcpu=v4:

  7: (7a) *(u64 *)(r0 +8) = 195936478
  BPF_ST imm must be 0 when storing to kptr at off=8
  ...
  EXPECTED   SUBSTR: 'invalid kptr access, R'

Pick the expected message with __BPF_FEATURE_ST, which clang and bpf-gcc
both define exactly when BPF_ST codegen is available - cpuv4 for clang,
and by default for bpf-gcc, whose default cpu is v4.

  bpf-gcc, before: #229/20 map_kptr/reject_scalar_store_to_kptr:FAIL
  bpf-gcc, after : #229/20 map_kptr/reject_scalar_store_to_kptr:OK

Two caveats worth noting:

- On a BPF_ST toolchain the test now only exercises the imm != 0 check
  and never reaches map_kptr_match_type(), so the scalar-vs-PTR_TO_BTF_ID
  rejection the test is named for is only covered by the non-ST builds.
  The imm path itself is already covered compiler-independently by
  verifier/map_kptr.c ("map_kptr: BPF_ST imm != 0").

- __BPF_FEATURE_ST says the compiler *can* emit BPF_ST, not that it will.
  The encoding also depends on the optimization level: clang -mcpu=v4 -O0
  still emits BPF_STX, which would send the #ifdef down the wrong branch
  and fail the test. Selftests always build BPF objects at -O2 so this
  does not bite today, but it is a latent failure mode if that changes.

Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20260807204434.1036279-2-vineet.gupta@linux.dev

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