[clang] [llvm] [BPF] Make -mcpu=v3 as the default (PR #107008)

[Yuval Deutscher via llvm-commits]

yuvald-sweet-security wrote:

> @yuvald-sweet-security Could you share which kernel you are used for above testing?

I ran this on my host machine which is Windows with WSL, kernel `6.6.75.1-microsoft-standard-WSL2`. However, I can see similar regressions on pretty much every testing VM that I have, for example here's Ubuntu 22.04 with kernel `6.8.0-1021-azure`:
```
$ sudo ~/veristat /tmp/test-v1.o --filter=trace_ret_vfs_writev_tail
Processing 'test-v1.o'...
File       Program                    Verdict  Duration (us)  Insns  States  Peak states
---------  -------------------------  -------  -------------  -----  ------  -----------
test-v1.o  trace_ret_vfs_writev_tail  success          23429  25015    1859         1728
---------  -------------------------  -------  -------------  -----  ------  -----------
Done. Processed 1 files, 0 programs. Skipped 1 files, 158 programs.

$ sudo ~/veristat /tmp/test-v3.o --filter=trace_ret_vfs_writev_tail
Processing 'test-v3.o'...
File       Program                    Verdict  Duration (us)  Insns  States  Peak states
---------  -------------------------  -------  -------------  -----  ------  -----------
test-v3.o  trace_ret_vfs_writev_tail  failure          98260  80350    6184         2738
---------  -------------------------  -------  -------------  -----  ------  -----------
Done. Processed 1 files, 0 programs. Skipped 1 files, 158 programs.
```

https://github.com/llvm/llvm-project/pull/107008