llvm: various config file fixes

[carlocab]

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Let's create config files that target aarch64 too. This is used as the
target by Python. See Homebrew/discussions#5778.

Also, use -isysroot instead of --sysroot so that our choice can be
overridden by setting SDKROOT. Fixes #197277.

[carlocab]

Cherry-picked onto #199909.

[madsmtm]

Also, use -isysroot instead of --sysroot so that our choice can be overridden by setting SDKROOT.

I don't think this will let users override by setting SDKROOT, it'll always prefer the value in -isysroot. But at least it also won't confusingly try to use different SDKs at compile and at link time, so definitely an improvement.

[carlocab]

Merged in #199909.