RFR: 8340804: doc/building.md update Xcode instructions to note that full install is required [v3]

[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

George Adams has updated the pull request incrementally with one additional commit since the last revision:

  Update building.md
  
  Co-authored-by: Erik Joelsson <37597443+erikj79 at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21154/files
  - new: https://git.openjdk.org/jdk/pull/21154/files/084cf7b2..32dd4677

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21154&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21154&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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