Update Linux and OS X build instructions

Update Linux and OS X build instructions

CoreClr (including mscorlib) and CoreFx can now be built using Mono on
both Linux and OS X; this change updates the instructions to reflect
that.  It also moves the instructions for the alternative option of
building the x-plat bits on Windows to cross-building.md.

This change also adds System.Globalization.Native, which is now
necessary.

At some point the FreeBSD documentation should also be updated to be
consistent with the Linux and OS X instructions; ideally, they should be
combined into one file, as discussed in https://github.com/dotnet/coreclr/issues/1590.

Commit migrated from https://github.com/dotnet/coreclr/commit/38787b0d6802eb6138b9ea1653d6fa8ed8063a42