Updated define constants to address bugzilla 24797 and add missing supported features.

Removed FX_NO_STRUCTURAL_EQUALITY, FX_NO_CUSTOMATTRIBUTEDATA as these
are both provided in mscorlib for monotouch/monodroid.
FX_NO_STRUCTURAL_EQUALITY actually causes an issue with array
comparisons within tuples, see:
https://bugzilla.xamarin.com/show_bug.cgi?id=24797

Removed FX_NO_BIGINT_CULTURE_PARSE as its already covered by
FX_NO_BIGINT

Added FX_ATLEAST_45, FSHARP_CORE_4_5 as .Net 4.5 features defined
within are supported by monotouch/monodroid