amd64: remove lfence after swapgs on syscall entry

[kib]

Summary

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.