[mono][aot] Optimize constrained calls made from gsharedvt methods. (#79339)

[mono][aot] Optimize constrained calls made from gsharedvt methods. (#79339)

The calls are of the form:
.constrained T_GSHAREDVT
callvirt <method>

Whenever T_GSHAREDVT is a reference or value type is only known at runtime.

Previously these were handled by passing the arguments to a JIT icall which
computed the target method and did a runtime invoke.

Added 2 optimizations:
* Precompute the data which depends only on the type and the method,
  store it in an rgctx slot and pass it to the JIT icall.
* Add a fastpath for simpler cases which makes an indirect call
  from generated code.