[MC][Target][XCOFF] Consolidate MCAsmInfo XCOFF defaults; NFC

The setting of `MCAsmInfo` properties for XCOFF got split between
`MCAsmInfoXCOFF` and `PPCXCOFFMCAsmInfo`. Except for the properties that
are dependent on the target information being passed via the
constructor, the properties being set in `PPCXCOFFMCAsmInfo` had no
fundamental reason for being treated as specific for XCOFF on PowerPC.
Indeed, the property that might be considered more specific to PowerPC,
`NeedsFunctionDescriptors`, was set in `MCAsmInfoXCOFF`.

XCOFF being specific to PowerPC anyway, this patch consolidates the
setting of the properties into `MCAsmInfoXCOFF` except for the cases
that are dependent on the information provided via the
`PPCXCOFFMCAsmInfo` constructor.

This patch also reorders the assignments to the fields to match the
declaration order in `MCAsmInfo`.