selftests/bpf: fix to avoid __msg tag de-duplication by clang

selftests/bpf: fix to avoid __msg tag de-duplication by clang

__msg, __regex and __xlated tags are based on
__attribute__((btf_decl_tag("..."))) annotations.

Clang de-duplicates such annotations, e.g. the following
two sequences of tags are identical in final BTF:

    /* seq A */            /* seq B */
    __tag("foo")           __tag("foo")
    __tag("bar")           __tag("bar")
    __tag("foo")

Fix this by adding a unique suffix for each tag using __COUNTER__
pre-processor macro. E.g. here is a new definition for __msg:

    #define __msg(msg) \
      __attribute__((btf_decl_tag("comment:test_expect_msg=" XSTR(__COUNTER__) "=" msg)))

Using this definition the "seq A" from example above is translated to
BTF as follows:

    [..] DECL_TAG 'comment:test_expect_msg=0=foo' type_id=X component_idx=-1
    [..] DECL_TAG 'comment:test_expect_msg=1=bar' type_id=X component_idx=-1
    [..] DECL_TAG 'comment:test_expect_msg=2=foo' type_id=X component_idx=-1

Surprisingly, this bug affects a single existing test:
verifier_spill_fill/old_stack_misc_vs_cur_ctx_ptr,
where sequence of identical messages was expected in the log.

Fixes: 537c3f66eac1 ("selftests/bpf: add generic BPF program tester-loader")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240820102357.3372779-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>