[PowerPC][Dwarf] Assign MMA register's dwarf register number to negative value

According to ELF V2 ABI, `0` should be the dwarf number of `r0`. Currently MMA's register also uses `0` as its dwarf number, this confuses `RegisterInfoEmitter` and generates wrong dwarf -> llvm mapping.
```
extern const MCRegisterInfo::DwarfLLVMRegPair PPCDwarfFlavour1Dwarf2L[] = {
  { 0U, PPC::VSRp31 },
```
This leads to wrong cfi output in https://reviews.llvm.org/D100290.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D103761