RFR: Update Readme with macOs specifics

[Brice Dutheil]

On Mon, 28 Mar 2022 17:04:14 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> The macOs libclang location as well as build output differ enough to warrant specific information. In particular the use of the XCode based path to find the libclang is not an easy find. The output also makes use of jpackage with `app-image` type, but the macOs _.app_ layout different enough on macOs.
>
> README.md line 19:
> 
>> 17: * `/Library/Developer/CommandLineTools/usr/` if using Command Line Tools
>> 18: * `/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/` if using XCode
>> 19: * `$(brew --prefix llvm)` if using the [LLVM install from Homebrew](https://formulae.brew.sh/formula/llvm#default)
> 
> FWIW, different package managers probably have different packages and folder structures.
> 
> I think the recommended way to get LLVM is still through the release page here: https://github.com/llvm/llvm-project/releases/tag/llvmorg-14.0.0 after which the tar can be extracted to any location, and `-Pllvm_home` pointed to that.

I have tested these combinations, although there's some differences in the actual library objects present in the above installs.

Although I must say I didn't tried jtreg.

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PR: https://git.openjdk.java.net/jextract/pull/10