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

The toolchain for powerpc spe can use uClibc-ng without thread support.
So we need the same fix as commit [1].

[1] fff68f75

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>