Pass __IntermediatesDir to msbuild in build.sh

Pass __IntermediatesDir to msbuild in build.sh

Currently, building coreclr x-plat fails if not running as root due to
the build trying to copy files to "/ToolRuntime".  The issue is that
$(IntermediatesOutputRootPath) is not set when running from build.sh,
and testruntime.targets in buildtools defines ToolRuntimePath as
"$(IntermediateOutputRootPath)ToolRuntime\".  The fix is to pass
__IntermediatesDir to msbuild in build_mscorlib, which results in
$(IntermediatesOutputRootPath) being properly set.

This change also adds a skipcoreclr flag to allow the user to skip the
CoreCLR part of the build, similar to the existing skipmscorlib flag.

Commit migrated from https://github.com/dotnet/coreclr/commit/18e80622347e41cc53ca2ffa880a19298edfd60d