Record public global definitions via field "prog_public" in AST.program.
For the moment, this field is ignored.
Add Genv.public_symbol operation.
Restrict pointer event values to public global names.
Update proofs accordingly.  PowerPC and ARM need updating.
Verification of the Unusedglob pass (removal of unreferenced static global definitions).  Assorted changes to ia32/Op.v.  PowerPC and ARM need updating.
Update PowerPC port.
Update the ARM port.
Merge pull request #2 from AbsInt/public-globals

Public globals