Add extra_exec_rustc_flags_triples, extra_exec_rustc_flags_exec_triples and extra_rustc_flags_exec_triples

[jesses-canva]

• extra_exec_rustc_flags_triples is just the exec version of extra_rustc_flags_triples. We don't strictly needed this but seemed like a strange omission (you may want to vary flags with the target triple in the exec configuration).

• extra_exec_rustc_flags_exec_triples and extra_rustc_flags_exec_triples are like extra_exec_rustc_flags_triples and extra_rustc_flags_triples, except instead of being indexed by the target triple, they are indexed by the execution triple.

We need this to pass -Clink-self-contained=no, which is only valid when rustc is running on x86_64-unknown-linux-gnu.