[release/6.0] [Libraries][Android] Fix alignment for padding System.Decimal (#58380)

[release/6.0] [Libraries][Android] Fix alignment for padding System.Decimal (#58380)

Fixes #49872

There is a difference in the current alignment of a decimal type on Android x86 (32bit) MONO_ABI_ALIGNOF (gpointer) which evaluates to 4 bytes and whats expected, which is 8 bytes. The behavior of MONO_ABI_ALIGNOF (gpointer) seems correct as gpointer is void* and 32-bit aligned pointers on a 32-bit platform makes sense. Instead, it's suspected that using MONO_ABI_ALIGNOF (gpointer) as the alignment is not correct for Android x86 if we are expecting an align of 8 bytes.

Attempting to remove the entire special casing for System Decimal in marshal.c didn't seem to work as noted in #49872 (comment)

This PR will adhere to the alignment value of 4 for Android x86, remove the special casing for System.Decimal because CoreCLR had removed their special casing, and modifies the expected values in the tests. https://github.com/dotnet/corefx/pull/30690/files?file-filters%5B%5D=.cs#r199212623 makes me suspect these tests have yet to be modified to fully accomodate Linux platforms.