aot: carry standalone runtime link metadata

[dank-openai]

Before this change, a runtime bundle could effectively say:

Here is the standalone archive.
Figure out the rest of the native link contract elsewhere.

After this change, the bundle says:

Here is the standalone archive.
Here is the producer-authored link manifest that belongs with it.
Consume them together.

Concretely, the generated runtime repo now exports:

• @<name>_runtime//:xls_aot_runtime
• @<name>_runtime//:xls_aot_runtime_file
• @<name>_runtime//:xls_aot_runtime_link_config_file

and the generated xlsynth_artifact_config.toml records both:

aot_runtime_path = "libxls_aot_runtime.a"
aot_runtime_link_config_path = "libxls_aot_runtime_link.toml"

The producer-owned TOML currently looks like this conceptually:

format_version = 1

[targets.macos]
system_libraries = ["c++"]
frameworks = ["CoreFoundation"]

[targets.linux]
system_libraries = ["c++", "c++abi", "unwind", "pthread", "dl"]
frameworks = []

Stack:

• ➡ aot: carry standalone runtime link metadata #59

⚠️ Part of a stack created by spr-multicommit. Do not merge manually using the UI - doing so may have unexpected results.