Auto merge of #1222 - MikaelUrankar:stack_t_bsd, r=gnzlbg

fix stack_t on FreeBSD

FreeBSD 10.x is EOL, in FreeBSD 11 and later, ss_sp is actually a void* [1]
dragonflybsd still uses c_char [2]

[1] https://svnweb.freebsd.org/base/releng/11.2/sys/sys/signal.h?revision=334459&view=markup#l438
[2] https://github.com/DragonFlyBSD/DragonFlyBSD/blob/master/sys/sys/signal.h#L339

should be committed with https://github.com/rust-lang/rust/pull/57810