[PPC64LE] Add a CALL_ELF macro to indicate use of the ELFv2 ABI.

Additional clarification from Uli for the background on _CALL_ELF:
"Historically GCC has provided various _CALL_... predefines depending on the
ABI currently being compiled for. (_CALL_SYSV,_CALL_AIXDESC, _CALL_DARWIN )
When we needed a new define for the current ABI, we decided on using _CALL_ELF
since the official name of the ABI is the OpenPower ElfV2 ABI, with the
current Linux ABI retro-actively being renamed the ELFv1 ABI
and so we decided on using _CALL_ELF to identify the Linux (+BSD etc.) ELF ABI,
with _CALL_ELF=1 for the v1 ABI and _CALL_ELF=2 for the v2 ABI.
(Note that this matches the gcc compiler switch -mabi=elfv1 vs. -mabi=elfv2)."

In code, a (_CALL_ELF==2) check will indicate when the ELFv2 ABI is
to be used.   This is the desired default for the PPC64 LE target.