RFR: 8250269: Replace ATTRIBUTE_ALIGNED with alignas [v15]

[Julian Waters]

> C++11 added the alignas attribute, for the purpose of specifying alignment on types, much like compiler specific syntax such as gcc's __attribute__((aligned(x))) or Visual C++'s __declspec(align(x)).
> 
> We can phase out the use of the macro in favor of the standard attribute. In the meantime, we can replace the compiler specific definitions of ATTRIBUTE_ALIGNED with a portable definition. We might deprecate the use of the macro but changing its implementation quickly and cleanly applies the feature where the macro is being used.
> 
> Note: With certain parts of HotSpot using ATTRIBUTE_ALIGNED so indiscriminately, this commit will likely take some time to get right
> 
> This will require adding the alignas attribute to the list of language features approved for use in HotSpot code. (Completed with [8297912](https://github.com/openjdk/jdk/pull/11446))

Julian Waters has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 15 additional commits since the last revision:

 - Merge branch 'openjdk:master' into alignas
 - alignas
 - Merge branch 'openjdk:master' into alignas
 - Merge branch 'openjdk:master' into alignas
 - Merge branch 'openjdk:master' into alignas
 - Merge branch 'openjdk:master' into alignas
 - Merge branch 'openjdk:master' into alignas
 - Merge branch 'openjdk:master' into alignas
 - Merge branch 'openjdk:master' into alignas
 - Merge branch 'openjdk:master' into alignas
 - ... and 5 more: https://git.openjdk.org/jdk/compare/efea724e...a621bb62

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11431/files
  - new: https://git.openjdk.org/jdk/pull/11431/files/b36f835c..a621bb62

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11431&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11431&range=13-14

  Stats: 12084 lines in 309 files changed: 5119 ins; 3210 del; 3755 mod
  Patch: https://git.openjdk.org/jdk/pull/11431.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11431/head:pull/11431

PR: https://git.openjdk.org/jdk/pull/11431