RFR: 8296904: Improve handling of macos xcode toolchain [v2]

[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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Improve options for xcode toolchain

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11113/files
  - new: https://git.openjdk.org/jdk/pull/11113/files/d9dcf5b2..dd808218

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11113&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11113&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 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