selftests/bpf: Enrich subtest_basic_usdt case in selftests to cover SIB handling logic

When using GCC on x86-64 to compile an usdt prog with -O1 or higher
optimization, the compiler will generate SIB addressing mode for global
array, e.g. "1@-96(%rbp,%rax,8)".

In this patch:
- enrich subtest_basic_usdt test case to cover SIB addressing usdt argument spec
  handling logic

Signed-off-by: Jiawei Zhao <phoenix500526@163.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250827053128.1301287-3-phoenix500526@163.com