[lldb/Core] Don't crash in GetSoftwareBreakpointTrapOpcode for unknown triples

This patch ensures we don't crash in GetSoftwareBreakpointTrapOpcode for
not-yet-supported architectures but rather continue with degraded
behavior.

I found the issue in the context of an invalid ArchSpec, which should be
handled further up the chain. In this patch I've also added an assert to
cover that, so we can still catch those issues.

Differential revision: https://reviews.llvm.org/D78588