Forward port r478722 | gerald | 2018-09-01 from lang/gcc7:

Disable the build/use of libssp/gets-chk since FreeBSD 12 and later
do not feature gets() any longer.

(I was planning to make this part of a routine update, alas last week's
snapshot was broken on i386-unknown-freebsd10.x and so will this week's
still.)

PR: 222796, 231066
Differential Revision: https://reviews.freebsd.org/D12298