[jdk11u-dev] RFR: 8360816: [11u] Use default value for ProgramFiles(x86) in GHA

[Antonio Vieiro]

A fix for [JDK-8360816](https://bugs.openjdk.org/browse/JDK-8360816). 

The `ProgramFiles(x86)` environment variable is not properly propagated to JTReg tests in JDK11, which makes the AOT compiler tests fail on the new `windows-2025` GHA runners, as manifested in the backport of [https://bugs.openjdk.org/browse/JDK-8358538](JDK-8358538).  

Propagating this environment variable to JTRegs may require creating devkits for Windows 2022, but previous attempts were discarded [JDK-8283723](https://bugs.openjdk.org/browse/JDK-8283723). 

This fix prints a warning message when this environment variable is not set, and then uses a default value (`C:\Program Files (x86)`) that makes it possible to run the AOT tests with these new `windows-2025` GHA runners.  If the `ProgramFiles(x86)` env variable is set the fix has no effect, ensuring the previous behaviour. If the proposed default value is incorrect the AOT tests will continue to fail as before.

The PR is on top of  https://github.com/openjdk/jdk11u-dev/pull/3052 to verify the fix works as intended with the new `windows-2025` GHA runners.

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

Depends on: https://git.openjdk.org/jdk11u-dev/pull/3052

Commit messages:
 - 8360816: [11u] Use default value for ProgramFiles(x86) in GHA

Changes: https://git.openjdk.org/jdk11u-dev/pull/3056/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3056&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360816
  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/3056.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3056/head:pull/3056

PR: https://git.openjdk.org/jdk11u-dev/pull/3056