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