RFR: 8371637: allocateNativeInternal sometimes return incorrectly aligned memory [v2]

[Harald Eilertsen]

> `jdk.internal.foreign.SegmentFactories::allocateNativeInternal` assumes that the underlying implementation of malloc aligns allocations on 16 byte boundaries for 64 bit platforms, and 8 byte boundaries on 32 bit platforms. So for any allocation where the requested alignment is less than or equal to this default alignment it makes no adjustment.
> 
> However, this assumption does not hold for all allocators. Specifically jemallc, used by libc on FreeBSD will align small allocations on 8 or 4 byte boundaries, respectively. This causes allocateNativeInternal to sometimes return memory that is not properly aligned when the requested alignment is exactly 16 bytes.
> 
> To make sure we honour the requested alignment when it exaclty matches the quantum as defined by MAX_MALLOC_ALIGN, this patch ensures that we adjust the alignment also in this case.
> 
> This should make no difference for platforms where malloc allready aligns on the quantum, except for a few unnecessary trivial calculations.
> 
> This work was sponsored by: The FreeBSD Foundation

Harald Eilertsen has updated the pull request incrementally with two additional commits since the last revision:

 - Second try to fix alignment for native segments
   
   Introducing a helper function as suggested by JornVernee to decide on
   the proper alignment based on the segment size.
   
   This work was sponsored by: The FreeBSD Foundation
   
   Co-authored-by: JornVernee
 - Test that native segments don't overlap
   
   This work was sponsored by: The FreeBSD Foundation

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28235/files
  - new: https://git.openjdk.org/jdk/pull/28235/files/0d1bb665..2b8266f6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28235&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28235&range=00-01

  Stats: 24 lines in 2 files changed: 17 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28235.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28235/head:pull/28235

PR: https://git.openjdk.org/jdk/pull/28235