[RISCV] Allow YAML file to control multilib selection (#98856) This changes the bare-metal driver logic such that it _always_ tries multilib.yaml if it exists, and it falls back to the hardwired/default RISC-V multilib selection only if a multilib.yaml doesn't exist. In contrast, the current behavior is that RISC-V can never use multilib.yaml, but other targets will try it if it exists. The flags `-march=` and `-mabi=` are exposed for multilib.yaml to match on. There is no attempt to help YAML file creators to duplicate the existing hard-wired multilib reuse logic -- they will have to implement it using `Mappings`. This should be backwards-compatible with existing sysroots, as multilib.yaml was previously never used for RISC-V, and the behavior doesn't change after this PR if the file doesn't exist.
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