assembler: support programs not starting at a defined label on the first instruction.
assembler: support programs not starting at a defined label on the first instruction.

(for example, fixes vCPU/clearscreen.vasm emitting code in the zero page)

kervinck/gigatron-rom,kervinck/gigatron-rom,kervinck/gigatron-rom,kervinck/gigatron-rom,kervinck/gigatron-rom,kervinck/gigatron-rom,kervinck/gigatron-rom