[unittest][DebugInfo/DWARF] Do not create dwarfgen::Generator if MCAsmBackend is missing

dwarfgen::Generator cannot be created if there is no asm backend for a
target. For example, if the default target triple is nvptx-nvidia-cuda,
some tests fail even after D98400, which added checks for most cases.
This patch extends the approach to the remaining cases.

Differential Revision: https://reviews.llvm.org/D116107