Deprecated and Obsolete #if blocks are not shared and generated separately

[chamons]

Both Deprecated and Obsolete generated the #if block like this:

#if __MACCATALYST__
        [Obsolete ("Starting with maccatalyst", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
#elif IOS
        [Obsolete ("Starting with ios11.0", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
#endif

However, if you have each, they don't share #if's

#if NET
        [UnsupportedOSPlatform ("maccatalyst")]
#if __MACCATALYST__
        [Obsolete ("Starting with maccatalyst", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
#endif
#if IOS
        [Obsolete ("Starting with ios11.0", DiagnosticId = "BI1234", UrlFormat = "https://github.com/xamarin/xamarin-macios/wiki/Obsolete")]
#endif
#else

We could improve this by changing ProcessObsolete and ProcessDeprecated to communicate which #ifs are needed and generate into the same #if.

There is a test with this issue tagged showing the current sub-ideal behavior that could be corrected as an example.