RFR: 8324838: test_nmt_locationprinting.cpp broken in the gcc windows build

[Julian Waters]

test_nmt_locationprinting.cpp defines method test_for_dead_c_heap_block unconditionally, but this method is only ever used on Linux. In the gcc windows build this fires an unused method warning and breaks the gtest suite. Since this is never used outside of Linux, make this method's definition only appear on Linux to avoid polluting code on other platforms

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Commit messages:
 - 8324838

Changes: https://git.openjdk.org/jdk/pull/17612/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17612&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324838
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17612.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17612/head:pull/17612

PR: https://git.openjdk.org/jdk/pull/17612