RFR: JDK-8292071: NMT: move MallocHeader to its own header and inline header checks

[Thomas Stuefe]

MallocHeader should be moved into its own header in preparation for further NMT changes.

Also, malloc header check routines - that are called on every free and realloc - should be inlined.

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Commit messages:
 - JDK-8292071-move-mallocheader-to-its-own-header

Changes: https://git.openjdk.org/jdk/pull/9801/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9801&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292071
  Stats: 572 lines in 5 files changed: 341 ins; 231 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9801.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9801/head:pull/9801

PR: https://git.openjdk.org/jdk/pull/9801