WebAssembly: handle private/internal globals.

Things of note:
 - Other linkage types aren't handled yet. We'll figure it out with dynamic linking.
 - Special LLVM globals are either ignored, or error out for now.
 - TLS isn't supported yet (WebAssembly will have threads later).
 - There currently isn't a syntax for alignment, I left it in a comment so it's easy to hook up.
 - Undef is convereted to whatever the type's appropriate null value is.
 - assert versus report_fatal_error: follow what other AsmPrinters do, and assert only on what should have been caught elsewhere.

llvm-svn: 246092