RFR: 8319197: Exclude hb-subset and hb-style from compilation

[Daniel Jeliński]

hb-subset and hb-style APIs are not used and not exported by libfontmanger. We can cut the compilation time by not compiling the unused files.

The added exclusions reduce the build time by ~1 minute (~8%) on my machine. This is with harfbuzz 8, with harfbuzz 7 the improvement was ~30 seconds.

Client libs tests continue to pass.

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Commit messages:
 - Add comment
 - Do not compile hb-subset files

Changes: https://git.openjdk.org/jdk/pull/16440/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16440&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319197
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16440.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16440/head:pull/16440

PR: https://git.openjdk.org/jdk/pull/16440