RFR: 8253779: Amalloc may be wasting space by overaligning

[Coleen Phillimore]

Thanks to @kimbarrett for noticing this. The alignment was changed to 64 bits for 32 bit platforms, but overalign for 64 bits platforms.  I changed this to BytesPerLong to cover both, since the long case is why it was changed on 32 bits in the first place in JDK-4526490.
I also removed Amalloc_D since I don't know what D stands for and it's the same as Amalloc_4.  That's not a great name either. I'm open to suggestions!
Tested with tier1-3.

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Commit messages:
 - Remove Amalloc_D because it's the same as Amalloc_4.
 - 8253779: Amalloc may be wasting space by overaligning

Changes: https://git.openjdk.java.net/jdk/pull/4732/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4732&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253779
  Stats: 34 lines in 7 files changed: 3 ins; 18 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4732.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4732/head:pull/4732

PR: https://git.openjdk.java.net/jdk/pull/4732