Fix System.Numerics.Vectors tests on iOS and other FullAOT targets

[akoeplinger]

Instead of using dynamic we can use explicit type checks which don't require runtime code generation.
Also fixed running just a subset of xunit tests on Apple targets in tests.mobile.targets.

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-numerics
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Issue Details

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Instead of using dynamic we can use explicit type checks which don't require runtime code generation.
Also fixed running just a subset of xunit tests on Apple targets in tests.mobile.targets.

Author:	akoeplinger
Assignees:	-
Labels:	area-System.Numerics, os-ios
Milestone:	-

[danmoseley]

This seems like it would be a great place to use the new generic math feature? @tannergooding

[tannergooding]

@danmoseley, yes, but it may also require more extensive changes to everything to allow it to work.

Probably worth an up-for-grabs work item.

[akoeplinger]

Had to disable a few tests on Android x86 since they triggered a runtime assert: #60347