Build an apphost with hostfxr and hostpolicy linked in (#35368)

Build an apphost with hostfxr and hostpolicy linked in (#35368)

* hostfxr: Build most of hostfxr as a static library

This is part of the work to create an apphost that bundles both hostfxr
and hostpolicy.  The main distinction between the static and shared
versions of hostfxr is that the static version contains a hostpolicy
resolver that references the hostpolicy symbols directly rather than
loading them from a DLL.

* hostpolicy: Build as a static library

This change is part of the work to enable an apphost that bundles both
hostfxr and hostpolicy.  There's no distinction between hostpolicy
that's built as a shared library and as a static library: the shared
library is built by linking an empty object file with the static
library.

* corehost: Allow linking of hostfxr and hostpolicy with apphost

Provide a hostfxr_iface class, that abstracts how the hostfxr functions called
by the early stage in the hosting layer is resolved.

* dotnet: Teach the muxer binary about hostfxr_iface

* apphost: Teach apphost about hostfxr_iface

This provides two implementations of hostfxr_iface: one for the static
apphost, which bundles hostfxr and hostpolicy, and another for the
conventional apphost, which loads them dynamically on startup.

* Add exports for hostfxr and policy

* Exports for unix

* EXPORTS_LINKER_OPTION

* use generateversionscript.awk from ENG

* Move fxr files out of static

* Fixes for Linux

* Fix for   win-x86

* move HEADERS next to SOURCES similarly to other files.

* PR feedback (simplifying hostpolicy_resolver::try_get_dir for static host)

* Publish static_apphost to Microsoft.NETCore.App.Host

* bind to entry points without probing, when in a static host.

* Add a test case

* renamed hostfxr_iface --> hostfxr_resolver_t

* renamed shared --> standalone

* rename static_apphost --> singlefilehost

* Signing exclusions for singlefilehost

* switched StaticHost tst to a different asset (mostly a copy of StandaloneApp)

Co-authored-by: Leandro Pereira <leandro.pereira@microsoft.com>
Co-authored-by: Swaroop Sridhar <swaroop.sridhar@microsoft.com>