RFR: 8281314: Rename Stack{Red,Yellow,Reserved,Shadow}Pages multipliers to "unit"

[Aleksey Shipilev]

This was a question during [JDK-8072070](https://bugs.openjdk.java.net/browse/JDK-8072070) review. The code in `StackOverflow::initialize_stack_zone_sizes()` multiplies `Stack{Red,Yellow,Reserved,Shadow}Pages` by "alignment", while that is actually just a "unit".


  product_pd(intx, StackYellowPages, \
          "Number of yellow zone (recoverable overflows) pages of size " \
          "4KB. If pages are bigger yellow zone is aligned up.") \
          range(MIN_STACK_YELLOW_PAGES, (DEFAULT_STACK_YELLOW_PAGES+5)) \


We can rename it for clarity.

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/7362/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7362&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281314
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7362.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7362/head:pull/7362

PR: https://git.openjdk.java.net/jdk/pull/7362