boehmgc: disable SOFT_VDB on powerpc64le for version 8.2.2

Upstream has not yet fixed the bug:

  ivmai/bdwgc#376
  ivmai/bdwgc#479

However there is a recommended workaround:

  ivmai/bdwgc#479 (comment)

This adds `CFLAGS_EXTRA=-DNO_SOFT_VDB` to the `makeFlags`, which
prevents direct accesses to `/proc` being used for tracking dirtied
pages (which must be rescanned):

  https://github.com/ivmai/bdwgc/blob/54522af853de28f45195044dadfd795c4e5942aa/include/private/gcconfig.h#L741

The collector will fall back to using mprotect() to trigger page
faults on writes to clean pages and maintain its own dirty bits,
which is slightly less efficient but (in this case) more reliable.
Unreliable page-dirtiness bits can lead to use-after-free()
corruption; this is not a situation where disabling the tests is a
good idea.

pkgs/development/libraries/boehm-gc/default.nix

@@ -30,6 +30,17 @@ stdenv.mkDerivation (finalAttrs: {
   ++ lib.optional enableMmap "--enable-mmap"
   ++ lib.optional enableLargeConfig "--enable-large-config";
 
+  # this stanza can be dropped when a release fixes this issue:
+  # https://github.com/ivmai/bdwgc/issues/376
+  makeFlags = lib.optionals (stdenv.hostPlatform.isPower64 &&
+                             lib.versionAtLeast finalAttrs.version "8.2.2")
+    [
+      # do not use /proc primitives to track dirty bits; see:
+      # https://github.com/ivmai/bdwgc/issues/479#issuecomment-1279687537
+      # https://github.com/ivmai/bdwgc/blob/54522af853de28f45195044dadfd795c4e5942aa/include/private/gcconfig.h#L741
+      "CFLAGS_EXTRA=-DNO_SOFT_VDB"
+    ];
+
   # `gctest` fails under emulation on aarch64-darwin
   doCheck = !(stdenv.isDarwin && stdenv.isx86_64);