[pmc2c] excempt Object.(pop|shift)_* error messages

Error: Missing manual_wb GC write barrier to Object.
Pmc2c/PMC/Object.pm special-cases :manual_wb default methods.
Skip those messages as the generated tail-call method calls
do it already. see src/pmc/object.c

Todo: Note that pmc2c could mark in special PMC handling pm's,
like Pmc2c/PMC/Object.pm those methods as :manual_wb by itself.