RFR: 8270179: Rename Amalloc_4

[Coleen Phillimore]

This renames Amalloc_4 to AmallocWords.  While I had to fix internal_malloc_4, which is a copy of Amalloc_4 (except with UseMallocOnly handling), I also made the change for
JDK-8270217 Fix Arena::Amalloc to check for overflow better
Tested with tier1 - all Oracle platforms and tier1-3 on linux-x64.

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Commit messages:
 - 8270179: Rename Amalloc_4

Changes: https://git.openjdk.java.net/jdk/pull/4750/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4750&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270179
  Stats: 94 lines in 16 files changed: 0 ins; 40 del; 54 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4750.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4750/head:pull/4750

PR: https://git.openjdk.java.net/jdk/pull/4750