Enable 'System.Private.CoreLib.dll' build on cross build mode (dotnet/coreclr#5320)
Currently, build.sh uses 'BuildOS' and 'BuildArch' to determine whether
it is possible to build 'System.Private.CorLib.dll'.
This commit revises build.sh to use 'HostOS' and 'HostArch' instead of
'BuildOS' and 'BuildArch' when it checkes whether it is possible to
build 'System.Private.CoreLib.dll', which allows to build
'System.Private.CoreLib.dll' in cross build mode.
Commit migrated from https://github.com/dotnet/coreclr/commit/
376b1fc601bdb4f37acfb7937634c928d745ef18