RFR: 8296904: Improve handling of macos xcode toolchain [v7]

[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 incrementally with one additional commit since the last revision:

  Update html file

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11113/files
  - new: https://git.openjdk.org/jdk/pull/11113/files/f066f764..df6d46f0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11113&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11113&range=05-06

  Stats: 20 lines in 1 file changed: 0 ins; 0 del; 20 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