RFR: 8250269: Replace ATTRIBUTE_ALIGNED with alignas [v6]

[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 incrementally with one additional commit since the last revision:

  ATTRIBUTE_ALIGNED(16)

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11431/files
  - new: https://git.openjdk.org/jdk/pull/11431/files/3f3a3625..ea968de2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11431&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11431&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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