[jdk11u-dev] RFR: 8257679: Improved unix compatibility layer in Windows build (winenv) [v10]

[George Adams]

> The main conflict is the lack of the WSL backport ([8215445: Enable building for Windows in WSL](https://github.com/openjdk/jdk/commit/46eb26b692ac7f9431d5b479028ec87e56707557)) Something which doesn't apply particularly cleanly and isn't required to achieve the MSYS2 backport that I'm trying to reach. There are also several Solaris specific edge cases that needed to be rewritten.
> 
> This PR will finally allow me to backport [8287724: Fix various issues with msys2](https://github.com/openjdk/jdk17u-dev/commit/2e21c87fac8d60c3c7ed7b2d0afae9abfc110359) cleanly which should be the final piece in the puzzle to enabling MSYS2 builds.
> 
> @RealCLanger / @GoeLin I suggest that this is run through the full SAP nightlies before this is merged.

George Adams has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - add toolchain changes
 - additional improvements
 - additional fixes
 - fixes
 - Backport d29c78da19ba78214efe9e7856cde30fdd9ba8ab

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Changes: https://git.openjdk.org/jdk11u-dev/pull/1278/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1278&range=09
  Stats: 4128 lines in 43 files changed: 1914 ins; 1876 del; 338 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1278.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1278/head:pull/1278

PR: https://git.openjdk.org/jdk11u-dev/pull/1278