amd64: remove lfence after swapgs on syscall entry

According to the description of SBSS issue at
https://software.intel.com/content/www/us/en/develop/articles/software-security-guidance/technical-documentation/speculative-behavior-swapgs-and-segment-registers.html
lfence after swapgs is needed only for the case when swapgs could be
speculatively executed. Since syscall entry, unlike exception and
interrupt entries, executes swapgs unconditionally, there is no
opportunity for speculation.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31682