Update threadasm.S to support ELFv2 on PPC64.

For the new Linux/PP64 LittleEndian boxes (POWER 8) a new ELF format was defined.
This commit updates the threadasm.S file to support ELFv2, too.
Merge pull request #988 from redstar/threadasm

Update threadasm.S to support ELFv2 on PPC64.