This file has a timestamp newer than these gcc-packages:

-rw-r--r-- 1 root root 15606060 Jan 25 13:41 gcc-7.3.0-i586-1.txz
-rw-r--r-- 1 root root 20446720 Jan 25 11:24 gcc-7.3.0-x86_64-1.txz

These were the first compilers to include mitigations for the
Spectre variant 2 vulnerability, specifically support for
-mindirect-branch=thunk-extern, allowing full mitigation of Spectre v2
in the kernel (when CONFIG_RETPOLINE is used).

Any gcc-compiled package in this Slackware tree with a timestamp newer
than this one has been has therefore been compiled using a compiler that
contains this mitigation.