[Local GC] Enable building the GC as a shared library (dotnet/coreclr#13137)

[Local GC] Enable building the GC as a shared library (dotnet/coreclr#13137)

* [Local GC] Build the GC using system headers

* Disable features to get the GC to build

* Get rid of the separate 'GC PAL' build

* Remove unused stuff

* Don't build gcenv.os.cpp when linking in a standalone gc

* Stub out CPUGroupInfo and NumaNodeInfo

* Stub out IsGCThread and friends

* Build the GC as a shared library :tada:

* Build, link, and run! :tada:

* Fix standalone GC build break

* Fixes where the GC's MethodTable and VM's MethodTable classes disagree

* Integrate a standalone GC into the CoreCLR build system (so it gets copied to the output folder). Re-enable some ifdef-ed out includes that are required for a non-standalone build of the GC.

* Bring changes to Unix and fix the Unix build. Implement some compiler intrinsic wrappers and alignment functions expected by the GC.

* Fix the Windows build

* 1. Code review feedback: use standard types for BitScanForward and
   BitScanForward64
2. Delete FEATURE_COM stuff from the build system, not needed for this
PR
3. Fix the Unix build

* Fix the Windows x86 build - the _BitScanForward64 intrinsic is not available when targeting 32-bit platforms

* Remove a number of things from gcenv.base.h that are not used

* Remove a linker workaround now that we are not linking a standalone GC into the runtime

* Remove dead code, make the lack of GC_PROFILING for standalone gc not break profiling on the non-standalone build

* Code review feedback - use add_library_clr and other cmake-related fixes

* Fix include indentation

* Remove some extraneous parameters to cmake functions (cmake is remarkably lenient...)

Commit migrated from https://github.com/dotnet/coreclr/commit/8acf6c9f7e33b1bfe77205cd2e61f2c60e8c00ce