rts: Teach getNumProcessors to return available processors

Previously we would report the number of physical processors, which can be quite wrong in a containerized setting. Now we rather return how many processors are in our affinity mask when possible.

I also refactored the code to prefer platform-specific since this will report logical CPUs instead of physical (using machdep.cpu.thread_count on Darwin and cpuset_getaffinity on FreeBSD).

Also cleans up some old Windows compatibility shims.

Fixes #14781 (closed).

docs/users_guide/8.12.1-notes.rst

@@ -17,7 +17,7 @@ Highlights
     digit improvements in runtime for inner loops.
 
     In the mean this improved runtime by about 0.8%. For details
-    see ticket #17823.
+    see ticket :ghc-ticket:`17823`.
 
 Full details
 ------------
@@ -95,7 +95,7 @@ Language
   effectively allows users to choose which variables can or can't be
   instantiated through visible type application. More information can be found
   here: :ref:`Manually-defining-inferred-variables`.
-  
+
 Compiler
 ~~~~~~~~
 
@@ -105,11 +105,18 @@ GHCi
 
 - The ``:script`` command now allows for file names that contain spaces to
   passed as arguments: either by enclosing the file names in double quotes or by
-  escaping spaces in file names with a backslash. (#18027)
+  escaping spaces in file names with a backslash. (:ghc-ticket:`18027`)
 
 Runtime system
 ~~~~~~~~~~~~~~
 
+- :rts-flag:`-N` without a count now tries to respect the number of processors
+  in the process's affinity mask, making GHC's behavior more predictable in
+  containerized settings (:ghc-ticket:`14781`).
+
+- Support for Windows Vista has been dropped. GHC-compiled programs now require
+  Windows 7 or later.
+
 Template Haskell
 ~~~~~~~~~~~~~~~~