[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.

0 files changed, 0 insertions, 0 deletions