RFR: 8296904: Improve handling of macos xcode toolchain [v9]

[Christoph Langer]

> With this PR I'd like to make it easier to use dedicated installations of Xcode on Mac OS for OpenJDK builds without having to switch the active Xcode globally via `xcode-select`.
> 
> I propose adding a new configure flag `--with-xcode-path` that takes the path to an Xcode installation. If the option is set, this path is expanded to a valid `TOOLCHAIN_PATH`.
> 
> Furthermore, I fix detection of xcodebuild and correctly setting the sysroot from the toolchain by moving `AC_SUBST(TOOLCHAIN_PATH)` before calling `BASIC_SETUP_XCODE_SYSROOT` and honoring `TOOLCHAIN_PATH` in `BASIC_SETUP_XCODE_SYSROOT` in the case when no devkit is specified. As I see it, this is a viable fix, even if not introducing `--with-xcode-path`.

Christoph Langer has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge branch 'master' into xcode-toolchain
 - Fix bullet list formatting in markdown and update html
 - Fix enumeration for html
 - Update html file
 - Add dots
 - Whitespace stuff
 - Minor polishing
 - Updating building.md doc
 - Merge branch 'master' into xcode-toolchain
 - Review suggestion
 - ... and 2 more: https://git.openjdk.org/jdk/compare/42c20374...2a5e4e44

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Changes: https://git.openjdk.org/jdk/pull/11113/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11113&range=08
  Stats: 86 lines in 3 files changed: 25 ins; 16 del; 45 mod
  Patch: https://git.openjdk.org/jdk/pull/11113.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11113/head:pull/11113

PR: https://git.openjdk.org/jdk/pull/11113