selftests/bpf: Fix usdt multispec failure with arm64/clang20 selftest build

When building the selftest with arm64/clang20, the following test failed:
  ...
  ubtest_multispec_usdt:PASS:usdt_100_called 0 nsec
  subtest_multispec_usdt:PASS:usdt_100_sum 0 nsec
  subtest_multispec_usdt:FAIL:usdt_300_bad_attach unexpected pointer: 0xaaaad82a2a80
  #471/2   usdt/multispec:FAIL
  #471     usdt:FAIL

But arm64/gcc11 built kernel selftests succeeded. Further debug found arm64/clang
generated code has much less argument pattern after dedup, but gcc generated
code has a lot more.

Check usdt probes with usdt.test.o on arm64 platform:

with gcc11 build binary:
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000054f8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp]
  stapsdt              0x00000031       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000005510, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 4]
  ...
  stapsdt              0x00000032       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000005660, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 60]
  ...
  stapsdt              0x00000034       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000070e8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 1192]
  stapsdt              0x00000034       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000007100, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 1196]
  ...
  stapsdt              0x00000032       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000009ec4, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[sp, 60]

with clang20 build binary:
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000009a0, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x9]
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000009b8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x9]
  ...
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000002590, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x9]
  stapsdt              0x0000002e       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x00000000000025a8, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x8]
  ...
  stapsdt              0x0000002f       NT_STAPSDT (SystemTap probe descriptors)
    Provider: test
    Name: usdt_300
    Location: 0x0000000000007fdc, Base: 0x0000000000000000, Semaphore: 0x0000000000000008
    Arguments: -4@[x10]

There are total 300 locations for usdt_300. For gcc11 built binary, there are
300 spec's. But for clang20 built binary, there are 3 spec's. The default
BPF_USDT_MAX_SPEC_CNT is 256, so bpf_program__attach_usdt() will fail for gcc
but it will succeed with clang.

To fix the problem, do not do bpf_program__attach_usdt() for usdt_300
with arm64/clang setup.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250624211802.2198821-1-yonghong.song@linux.dev