package/libffi: disable use of static exec trampolines

TestGst1Python test segfault since the libffi bump to 3.4.2.

Apply the same fix from Yocto [1] disabling static exec trampolines.

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/1522848331

[1] http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=dadfef3950fae4e93ce4c13ab91a2a7f41b3702e



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>