[release/6.0] [mini] Use alloca for runtime_invoke retval buffer (#59006)

[release/6.0] [mini] Use alloca for runtime_invoke retval buffer (#59006)

* [mini] Use alloca for runtime_invoke retval buffer

Fixes https://github.com/dotnet/runtime/issues/58957
Related to https://github.com/dotnet/runtime/pull/58215 which was attempting to
fix https://github.com/dotnet/runtime/issues/58190

* Set initial return buffer size to TARGET_SIZEOF_VOID_P

In the common case we use the return buffer just to hold a pointer to
the return value

Co-authored-by: Aleksey Kliger <alklig@microsoft.com>
Co-authored-by: Aleksey Kliger <aleksey@lambdageek.org>