selftests/bpf: Fix pyperf180 compilation failure with clang18

With latest clang18 (main branch of llvm-project repo), when building bpf selftests,
[~/work/bpf-next (master)]$ make -C tools/testing/selftests/bpf LLVM=1 -j

The following compilation error happens:
fatal error: error in backend: Branch target out of insn range
...
Stack dump:
0. Program arguments: clang -g -Wall -Werror -D__TARGET_ARCH_x86 -mlittle-endian
-I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include
-I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -I/home/yhs/work/bpf-next/tools/include/uapi
-I/home/yhs/work/bpf-next/tools/testing/selftests/usr/include -idirafter
/home/yhs/work/llvm-project/llvm/build.18/install/lib/clang/18/include -idirafter /usr/local/include
-idirafter /usr/include -Wno-compare-distinct-pointer-types -DENABLE_ATOMICS_TESTS -O2 --target=bpf
-c progs/pyperf180.c -mcpu=v3 -o /home/yhs/work/bpf-next/tools/testing/selftests/bpf/pyperf180.bpf.o
1. <eof> parser at end of file
2. Code generation
...

The compilation failure only happens to cpu=v2 and cpu=v3. cpu=v4 is okay
since cpu=v4 supports 32-bit branch target offset.

The above failure is due to upstream llvm patch [1] where some inlining behavior
are changed in clang18.

To workaround the issue, previously all 180 loop iterations are fully unrolled.
The bpf macro __BPF_CPU_VERSION__ (implemented in clang18 recently) is used to avoid
unrolling changes if cpu=v4. If __BPF_CPU_VERSION__ is not available and the
compiler is clang18, the unrollng amount is unconditionally reduced.

[1] https://github.com/llvm/llvm-project/commit/1a2e77cf9e11dbf56b5720c607313a566eebb16e

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20231110193644.3130906-1-yonghong.song@linux.dev

selftests/bpf: add pyperf scale test

Add a snippet of pyperf bpf program used to collect python stack traces
as a scale test for the verifier.

At 189 loop iterations llvm 9.0 starts ignoring '#pragma unroll'
and generates partially unrolled loop instead.
Hence use 50, 100, and 180 loop iterations to stress test.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>