target: default to the medium code model on LoongArch targets

The Rust LoongArch targets have been using the default LLVM code model
so far, which is "small" in LLVM-speak and "normal" in LoongArch-speak.
As described in the "Code Model" section of LoongArch ELF psABI spec
v20231219 [1], one can only make function calls as far as ±128MiB with
the "normal" code model; this is insufficient for very large software
containing Rust components that needs to be linked into the big text
section, such as Chromium.

Because:

* we do not want to ask users to recompile std if they are to build
  such software,
* objects compiled with larger code models can be linked with those
  with smaller code models without problems, and
* the "medium" code model is comparable to the "small"/"normal" one
  performance-wise (same data access pattern; each function call
  becomes 2-insn long and indirect, but this may be relaxed back into
  the direct 1-insn form in a future LLVM version), but is able to
  perform function calls within ±128GiB,

it is better to just switch the targets to the "medium" code model,
which is also "medium" in LLVM-speak.

[1]: https://github.com/loongson/la-abi-specs/blob/v2.30/laelf.adoc#code-models

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