Avoid crash that happens for some users in qsb, qmake, and
potentially other Qt tools when ran under sandbox leading
to build failures for qtdeclarative and other packages.

Former fix involved replacing 4096 by SIGSTKSZ but
bug #918664 shown that this may be insufficient so this
tries 32k instead.

Update: after https://bugs.gentoo.org/923013 and a fixed
sandbox been stabilized for some time, it should in theory
be possible to drop this

https://bugs.gentoo.org/908809
https://bugs.gentoo.org/908816
https://bugs.gentoo.org/913493
https://bugs.gentoo.org/915695
https://bugs.gentoo.org/918664
https://codereview.qt-project.org/c/qt/qtbase/+/513140
--- a/src/3rdparty/forkfd/forkfd_linux.c
+++ b/src/3rdparty/forkfd/forkfd_linux.c
@@ -158,5 +158,5 @@
 int system_vforkfd(int flags, pid_t *ppid, int (*childFn)(void *), void *token, int *system)
 {
-    __attribute__((aligned(64))) char childStack[SIGSTKSZ];
+    __attribute__((aligned(64))) char childStack[32768];
     pid_t pid;
     int pidfd;