X86/AArch64/ARM: Factor out common sincos_stret logic; NFCI

Note:
- X86ISelLowering: setLibcallName(SINCOS) was superfluous as
  InitLibcalls() already does it.
- ARMISelLowering: Setting libcallnames for sincos/sincosf seemed
  superfluous as in the darwin case it wouldn't be used while for all
  other cases InitLibcalls already does it.

llvm-svn: 321036