`let` syntax for c++ backend.  fixes regtests/asm/asm_hello.py
`let` syntax for c++ backend.  fixes regtests/asm/asm_hello.py

rusthon/Rusthon,secureosv/pythia,pombredanne/Rusthon,secureosv/pythia,rootfs/Rusthon,tempbottle/Rusthon,tempbottle/Rusthon,kustomzone/Rusthon,pombredanne/Rusthon,pombredanne/Rusthon,secureosv/pythia,kustomzone/Rusthon,secureosv/pythia,kustomzone/Rusthon,kustomzone/Rusthon,rusthon/Rusthon,rootfs/Rusthon,tempbottle/Rusthon,rootfs/Rusthon,pombredanne/Rusthon,rusthon/Rusthon,pombredanne/Rusthon,kustomzone/Rusthon,rootfs/Rusthon,rusthon/Rusthon,tempbottle/Rusthon,tempbottle/Rusthon,rootfs/Rusthon,rusthon/Rusthon,secureosv/pythia