[config.toml] should not assume `ar` uses the same prefix as `cc` for target

[nagisa]

If you specify cc = "prefix-cc", then it will explicitly require "prefix-ar" to also exist, without any way to override it. Should either be overridable, or not assumed.

[metux]

In general, compiler(-frontends) should honor the $AR environment variable, and do not attempt any guessing whatsoever, when it's set (note: this is the command line, not the binary name) - distro packaging systems need that.

[bjorn3]

The ar path has been separately overridable since #45191:

rust/config.toml.example

Line 657 in 10b3a0d

#ar = "ar" (path)