Have ARMBaseRegisterInfo::getCallPreservedMask return the 'correct' mask for the GHC calling convention.

This is purely academic because GHC calls are always tail calls so the register mask will never be used; however, this change makes the code clearer and brings the ARM implementation of the GHC calling convention in line with the X86 implementation. Also, it might save someone else some time trying to figuring out what is happening...