RFR: 8139457: Array bases are aligned at HeapWord granularity [v6]

[Roman Kennke]

> See [JDK-8139457](https://bugs.openjdk.org/browse/JDK-8139457) for details.
> 
> Basically, when running with -XX:-UseCompressedClassPointers, arrays will have a gap between the length field and the first array element, because array elements will only start at word-aligned offsets. This is not necessary for smaller-than-word elements.
> 
> Also, while it is not very important now, it will become very important with Lilliput, which eliminates the Klass field and would always put the length field at offset 8, and leave a gap between offset 12 and 16.
> 
> Testing:
>  - [x] runtime/FieldLayout/ArrayBaseOffsets.java (x86_64, x86_32, aarch64, arm, riscv, s390)
>  - [x] bootcycle (x86_64, x86_32, aarch64, arm, riscv, s390)
>  - [x] tier1 (x86_64, x86_32, aarch64, riscv)
>  - [x] tier2 (x86_64, aarch64, riscv)
>  - [x] tier3 (x86_64, riscv)

Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:

 - Clean cast warning from size_t to int
 - Clear leading 32bits in Z array allocator

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11044/files
  - new: https://git.openjdk.org/jdk/pull/11044/files/ca491adc..77e2ace4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11044&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11044&range=04-05

  Stats: 12 lines in 2 files changed: 9 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11044.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11044/head:pull/11044

PR: https://git.openjdk.org/jdk/pull/11044