amd64/linux*: add required header to get the constant value

Otherwise asm silently interpret it as the external global symbol.

Approved by: so
Security: FreeBSD-SA-21:11.smap
Security: CVE-2021-29628
Reported by: bz
Sponsored by: The FreeBSD Foundation
Fixes: 91aae953cb80

(cherry picked from commit a59f0285377aa3d61cccda64e9ade126ecb3d2d9)
(cherry picked from commit b25bb77861640456a77fbf6f34cac4a3b69f0e5f)