package/gcc: disable split-stack for non-thread builds

Since commit [1], the autobuilder script generates fully random
configurations that can trigger configurations that were not tested
before.

Here, the toolchain build with uClibc-ng without threads support
(BR2_PTHREADS_NONE=y) fails to build due to a missing pthread.h
header:

../../../libgcc/generic-morestack-thread.c:42:10: fatal error: pthread.h: No such file or directory
   42 | #include <pthread.h>

This issue was actually fixed by this commit [2] adding a patch for
gcc 4.8, 4.9, 5.3. But it get lost when gcc 6 was added to Buildroot [3].

Since then the issue was present in Buildroot but has not been noticed.

[1] https://git.buildroot.net/buildroot-test/commit/?id=27b18dcb1686a98ce718b6a816e98f8505957a6c
[2] 2631219f
[3] 519d83bf

Fixes:
http://autobuild.buildroot.org/results/5ec/5ec9eefacd27ef4fa73066013188796b43a30428
https://bugs.busybox.net/show_bug.cgi?id=8766



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>