[Local GC] Provide an implementation of GCToOSInterface for Unix-like platforms ...

[Local GC] Provide an implementation of GCToOSInterface for Unix-like platforms (dotnet/coreclr#8976)

* Add way to build with FEATURE_STANDALONE_GC from build.sh
* Make CMake changes to build the GC 'PAL' as its own build target (to avoid -nostdinc).
In addition, introduce a "GC PAL" that provides an implementation of
GCToOSInterface on Unix-like platforms, for use with
FEATURE_STANDALONE_GC.

Commit migrated from https://github.com/dotnet/coreclr/commit/bbd32c484f2d00a566217e52bddb6960c2ec0b22