[Clang][BPF] Add __BPF_CPU_VERSION__ macro (#71856)

Sometimes bpf developer might want to develop different codes
based on particular cpu versioins. For example, cpu v1/v2/v3
branch target is 16bit while cpu v4 branch target is 32bit,
thus cpu v4 allows more aggressive loop unrolling than cpu v1/v2/v3
(see [1] for a kernel selftest failure due to this).
We would like to maintain aggressive loop unrolling for cpu v4
while limit loop unrolling for earlier cpu versions.
Another example, signed divide also only available with cpu v4.

Actually, adding cpu specific macros are fairly common
in llvm. For example, x86 has maco like 'i486', '__pentium_mmx__', etc.
AArch64 has '__ARM_NEON', '__ARM_FEATURE_SVE', etc.

This patch added __BPF_CPU_VERSION__ macro. Current possible values
are 0/1/2/3/4. The following are the -mcpu=... to __BPF_CPU_VERSION__
mapping:
```
       cpu                  __BPF_CPU_VERSION__
       no -mcpu=<...>       1
       -mcpu=v1             1
       -mcpu=v2             2
       -mcpu=v3             3
       -mcpu=v4             4
       -mcpu=generic        1
       -mcpu=probe          0
```
    
This patch also added some macros for developers to identify some cpu
insn features:
```
      feature macro               enabled in which cpu
      __BPF_FEATURE_JMP_EXT       >= v2
      __BPF_FEATURE_JMP32         >= v3
      __BPF_FEATURE_ALU32         >= v3
      __BPF_FEATURE_LDSX          >= v4
      __BPF_FEATURE_MOVSX         >= v4
      __BPF_FEATURE_BSWAP         >= v4
      __BPF_FEATURE_SDIV_SMOD     >= v4
      __BPF_FEATURE_GOTOL         >= v4
      __BPF_FEATURE_ST            >= v4
```    
[1]
https://lore.kernel.org/bpf/3e3a8a30-dde0-43a1-981e-2274962780ef@linux.dev/