RFR: 8255128: linux x86 build failure with libJNIPoint.c

[Jorn Vernee]

Guard libJNIPoint.c impl in `#ifdef _LP64` block

This fixes a 32-bit build error

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Commit messages:
 - Guard libJNIPoint.c impl in #ifdef _LP64 block

Changes: https://git.openjdk.java.net/jdk/pull/956/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=956&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255128
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/956.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/956/head:pull/956

PR: https://git.openjdk.java.net/jdk/pull/956