[ORC] Add a generic bootstrap key-value store to SimpleRemoteEPC.

[ORC] Add a generic bootstrap key-value store to SimpleRemoteEPC.

SimpleRemoteEPC already included a "bootstrap symbols map" that could be used
to communicate the addresses of symbols needed for JIT bootstrap. The new
bootstrap map can be used to communicate arbitrary bootstrap values (encoded as
SPS buffers).

The bootstrap symbols map is kept as distinct becasue bootstrap symbols are
significant, and having a known value type for them allows for better debug
logging (we know how to render the values) and tooling (e.g. utils for adding
all bootstrap symbols to a JITDylib).