rustbuild: allow overriding list of LLVM targets to build

A new option is introduced under the `[llvm]` section of `config.toml`,
`targets`, for overriding the list of LLVM targets to build support for.
The option is passed through to LLVM configure script. Also notes are
added about the implications of (ab)using the option; since the default
is not changed, and users of the option are expected to know what
they're doing anyway (as every porter should), the impact should be
minimal.

Fixes #38200.

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