* [mono] Optimize generic sharing for generic methods.

  Previously, rgctx entries where stored either in the class rgctx
or the method rgctx in linked structures, and accessed using
rgctx trampolines (for non-llvmonly configuration), or
inline code and fallback C code (for llvmonly configuration).
However, if a method has an mrgctx parameter, all the rgctx entries
can be stored as an array in the mrgctx and accessed using a simple
load.
One complication is that the mrgctx might need to be allocated before
the method is compiled/loaded, so the rgctx entries need to be stored
in a separate array, and the array needs to be initialized on
demand.

* Add an 'entries' field to MonoMethodRuntimeGenericContext which stores
the rgctx entries.
* Add a MonoGSharedMethodInfo structure which stores the information
needed to initialize a MonoMethodRuntimeGenericContext, i.e. the
number of rgctx entries and their contents.
* Add a MONO_PATCH_INFO_GSHARED_METHOD_INFO patch type to refer
to the previous structure.
* Add a mini_init_method_rgctx () JIT icall, which initializes
an mrgctx if needed and generate code in the prolog of gshared methods
to call it.

* [mono] Pass an mrgctx to static/valuetype gshared methods.

Previously, these methods were passed a vtable. Passing the mrgctx
instead simplifies some runtime code and allows smaller/faster access to
rgctx entries.

* Add rgctx trampolines in get_ftnptr_for_method ().

* [mono][wasm] Avoid AOTing Microsoft.CodeAnalysis.dll as well.