[jdk8u-dev] RFR: 8369226: GHA: Switch to MacOS 15 [v3]

[Andrew John Hughes]

> This is a backport of [JDK-8369226](https://bugs.openjdk.org/browse/JDK-8369226) from 11u that upgrades from macos-13 to macos-15-intel, and XCode from 14.3.1 to 16.4, as MacOS 13 runners are now unavailable.
> 
> Due to the different structure of the GHA support in 8u, and the absence of aarch64 on MacOS, this is a backport in spirit rather than a 1:1 relationship, with the same values being changed in their different locations in the 8u `submit.yml`.

Andrew John Hughes has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - Merge remote-tracking branch 'dev/master' into JDK-8369226
 - Revert LOG_LEVEL=debug additions
 - Backport 1b0f612e5e08cfa1581cf22433b36b5f075c0f23
 - Print spec files on all POSIX platforms
 - Add LOG_LEVEL=debug to all GHA builds
 - Whitespace cleanup
 - 8374899: [8u] Fully handle clang as the toolchain in flags.m4

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

Changes: https://git.openjdk.org/jdk8u-dev/pull/738/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=738&range=02
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/738.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/738/head:pull/738

PR: https://git.openjdk.org/jdk8u-dev/pull/738