RFR: 8377905: gcc.md included with every build

[Mikael Vidstedt]

JDK-8377509 added the gcc.md license file. Unfortunately the file is unconditionally in the built JDK regardless of the toolchain used. This change updates the build logic to only include the file when relevant.

Testing: tier1, builds-tier[2-5], manual verification to ensure the file is only included where expected.

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

Commit messages:
 - 8377905: gcc.md included with every build

Changes: https://git.openjdk.org/jdk/pull/29853/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29853&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377905
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29853.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29853/head:pull/29853

PR: https://git.openjdk.org/jdk/pull/29853