[BPF] Make sure ALU32 feature is set in MCSubtargetInfo for mcpu=v3

`BPF.td` is used to generate (among other things) `MCSubtargetInfo`
setup function for BPF target.
Specifically, the `BPFGenSubtargetInfo.inc` file:

    enum {
      ALU32 = 0,
      ...
    };
    ...
    extern const llvm::SubtargetSubTypeKV BPFSubTypeKV[] = {
      { "generic", { { { 0x0ULL, ... } } }, ... },
      { "probe",   { { { 0x0ULL, ... } } }, ... },
      { "v1",      { { { 0x0ULL, ... } } }, ... },
      { "v2",      { { { 0x0ULL, ... } } }, ... },
      { "v3",      { { { 0x1ULL, ... } } }, ... },
    };
    ...
    static inline MCSubtargetInfo *createBPFMCSubtargetInfoImpl(...) {
      return new BPFGenMCSubtargetInfo(..., BPFSubTypeKV, ...);
    }

The `SubtargetSubTypeKV` is defined in `MCSubtargetInfo.h` as:

    /// Used to provide key value pairs for feature and CPU bit flags.
    struct SubtargetSubTypeKV {
      const char *Key;                      ///< K-V key string
      FeatureBitArray Implies;              ///< K-V bit mask
      FeatureBitArray TuneImplies;          ///< K-V bit mask
      const MCSchedModel *SchedModel;
      ...
    }

The first bit array specifies features enabled by default for a
specific CPU. This commit makes sure that this information is
communicated to `tablegen` and correct `BPFSubTypeKV` table is
generated. This allows tools like `objdump` to detect available
features when `--mcpu` flag is specified.

Differential Revision: https://reviews.llvm.org/D148037