[Mono] UnsafeAccessor: Support ambiguous method match

[fanyang-mono]

Fixes#89098

This PR adjusted Mono's method lookup behavior for UnsafeAccessor to match with CoreCLR. It conducted two rounds of lookup.

• Relaxed match. Look for methods but ignore custom modifiers. If there are less than or equal to one match, return. If there are more than one match, enter exact match.
• Exact match. Look for methods matching exactly. If found, return the exact match. If not found, throw AmbiguousMatchException.

Special notes for function pointer calling convention:
If there is a single calling convention defined (i.e., Cdecl or Stdcall) then these values are not defined as custom modifiers, but instead encoded in the ECMA bits for the function pointer. They become custom modifiers, when there are more than one calling conventions defined. For example,

public unsafe class C {
public void M(delegate* unmanaged[Cdecl]<void> p) {}
public void N(delegate* unmanaged[Cdecl, SuppressGCTransition]<void> p) {}
}

The corresponding IL code for methods N and M is

 .method public hidebysig instance void M (
method unmanaged cdecl void *() p
) cil managed {
// Method begins at RVA 0x2069
// Code size 2 (0x2)
.maxstack 8
IL_0000: nop
IL_0001: ret
} // end of method C::M
.method public hidebysig instance void N (
method unmanaged void modopt([System.Runtime]System.Runtime.CompilerServices.CallConvSuppressGCTransition) modopt([System.Runtime]System.Runtime.CompilerServices.CallConvCdecl) *() p
) cil managed {
// Method begins at RVA 0x2069
// Code size 2 (0x2)
.maxstack 8
IL_0000: nop
IL_0001: ret
} // end of method C::N

[lambdageek]

LGTM.

My one suggestion is to move all the unsafe accessor stuff to unsafe-accessor.c because now it doesn't depend on the loader's lookup methods.

(It would be nice to make them share, but I think this lookup stuff is kind of fragile, and if we're going to be clarifying how UnsafeAccessor method lookup is supposed to work in .net 9, we'll have to tweak it probably, anyway. So separate file is better, I think)

[fanyang-mono]

Test failures are unrelated and have been reported.