RFR: 8340804: doc/building.md update Xcode instructions to note that full install is required

[George Adams]

Since JDK17 there has been a dependency on metal in the macOS builds which is only available as part of the full Xcode IDE (rather than command-line tools). We currently recommend only installing the command-line tools in the building.md doc which is incorrect.

See https://github.com/openjdk/jdk/blob/e1c4d3039f6b5106ce3f65d50f607eacc2a8d168/make/autoconf/toolchain.m4#L681

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Commit messages:
 - 8340804: doc/building.md update Xcode instructions to note that full install is required

Changes: https://git.openjdk.org/jdk/pull/21154/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340804
  Stats: 11 lines in 2 files changed: 0 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21154.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21154/head:pull/21154

PR: https://git.openjdk.org/jdk/pull/21154