bgen must be executed with the system mono, not bmac-mobile-mono, and without
the MONO_PATH variable set.

The new handling went in with mono/mono#4501.

I also noticed that WatchOS was missing a target for System.Drawing.Primitives.dll, so I added that.

…uded in linked apps. Fixes #56307 and #56308.

System.dll is now completely linked away unless the app actually uses any
System.dll API.

This is the change that caused this to change: mono/mono@4960d5d

Previously the following types would always be kept by the linker:

```
$ monodis --typedef System.dll
Typedef Table
1: (null) (flist=1, mlist=1, flags=0x0, extends=0x0)
2: ObjCRuntime.INativeObject (flist=1, mlist=1, flags=0xa0, extends=0x0)
3: Mono.Net.CFObject (flist=1, mlist=2, flags=0x100000, extends=0x5)
4: Mono.Net.CFArray (flist=4, mlist=19, flags=0x100, extends=0xc)
5: Mono.Net.CFNumber (flist=5, mlist=32, flags=0x100100, extends=0xc)
6: Mono.Net.CFRange (flist=5, mlist=41, flags=0x100108, extends=0x25)
7: Mono.Net.CFString (flist=7, mlist=42, flags=0x100100, extends=0xc)
8: Mono.Net.CFData (flist=8, mlist=53, flags=0x100100, extends=0xc)
9: Mono.Net.CFDictionary (flist=8, mlist=63, flags=0x0, extends=0xc)
10: Mono.Net.CFMutableDictionary (flist=10, mlist=75, flags=0x100100, extends=0x24)
11: Mono.Net.CFUrl (flist=10, mlist=80, flags=0x100100, extends=0xc)
12: Mono.Net.CFRunLoop (flist=10, mlist=83, flags=0x100100, extends=0xc)
13: Mono.Net.CFBoolean (flist=10, mlist=94, flags=0x100, extends=0x5)
14: Mono.AppleTls.SecCertificate (flist=13, mlist=106, flags=0x100100, extends=0x5)
15: Mono.AppleTls.SecIdentity (flist=14, mlist=122, flags=0x100, extends=0x5)
16: Mono.AppleTls.SecIdentity/ImportOptions (flist=19, mlist=134, flags=0x100105, extends=0x5)
17: Mono.AppleTls.SecKey (flist=19, mlist=134, flags=0x100100, extends=0x5)
18: Mono.AppleTls.SecStatusCode (flist=21, mlist=141, flags=0x100, extends=0x69)
19: Mono.AppleTls.SecTrustResult (flist=395, mlist=141, flags=0x100, extends=0x69)
20: Mono.AppleTls.SecImportExport (flist=404, mlist=141, flags=0x100100, extends=0x5)
21: Mono.AppleTls.SecImportExport/<>c (flist=404, mlist=144, flags=0x102103, extends=0x5)
22: Mono.AppleTls.SecPolicy (flist=406, mlist=147, flags=0x100100, extends=0x5)
23: Mono.AppleTls.SecTrust (flist=407, mlist=154, flags=0x100100, extends=0x5)
24: System.Security.Cryptography.OidGroup (flist=408, mlist=174, flags=0x101, extends=0x69)
25: System.Security.Cryptography.Oid (flist=420, mlist=174, flags=0x100101, extends=0x5)
26: System.Security.Cryptography.CAPI (flist=423, mlist=176, flags=0x100180, extends=0x5)
27: System.Security.Cryptography.AsnEncodedData (flist=423, mlist=178, flags=0x100101, extends=0x5)
28: System.Security.Cryptography.X509Certificates.X509Utils (flist=424, mlist=179, flags=0x100100, extends=0x5)
29: System.Security.Cryptography.X509Certificates.PublicKey (flist=424, mlist=181, flags=0x100101, extends=0x5)
30: System.Security.Cryptography.X509Certificates.X509Certificate2 (flist=429, mlist=188, flags=0x102101, extends=0x51)
31: System.Security.Cryptography.X509Certificates.X509Certificate2Impl (flist=431, mlist=204, flags=0x100080, extends=0x55)
32: System.Security.Cryptography.X509Certificates.X509CertificateCollection (flist=431, mlist=209, flags=0x102101, extends=0x6d)
33: System.Security.Cryptography.X509Certificates.X509CertificateCollection/X509CertificateEnumerator (flist=431, mlist=212, flags=0x100102, extends=0x5)
34: System.Security.Cryptography.X509Certificates.X509Helper2 (flist=432, mlist=217, flags=0x100180, extends=0x5)
35: <PrivateImplementationDetails> (flist=432, mlist=218, flags=0x100, extends=0x5)
36: <PrivateImplementationDetails>/__StaticArrayInitTypeSize=9 (flist=433, mlist=219, flags=0x113, extends=0x25)
```

Some of the above types from System.dll implemented ObjCRuntime.INativeObject
(from System.dll), which our linker detected as implementing
ObjCRuntime.INativeObject (from Xamarin.iOS.dll), so these types were treated
as custom NSObject subclasses, and the MarkNSObjects linker step would mark
them (which would in turn cause all the other types in the list to be marked).

With that change, these types now implement ObjCRuntimeInternal.INativeObject,
and the linker does not treat them as custom NSObject subclasses anymore.

I think the new behavior is correct: these types do not actually inherit from
the real NSObject/INativeObject, so the linker should not treat them as such.

This may run into different bugs because the linker might now remove more
stuff than before, but that would be a different issue.

This means that the fix is to modify these tests accordingly.

https://bugzilla.xamarin.com/show_bug.cgi?id=56307
https://bugzilla.xamarin.com/show_bug.cgi?id=56308