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

sys/amd64/amd64/exception.S

@@ -524,7 +524,6 @@ prot_addrf:
 	SUPERALIGN_TEXT
 IDTVEC(fast_syscall_pti)
 	swapgs
-	lfence
 	cmpq	$~0,PCPU(UCR3)
 	je	fast_syscall_common
 	movq	%rax,PCPU(SCRATCH_RAX)
@@ -535,7 +534,6 @@ IDTVEC(fast_syscall_pti)
 	SUPERALIGN_TEXT
 IDTVEC(fast_syscall)
 	swapgs
-	lfence
 fast_syscall_common:
 	movq	%rsp,PCPU(SCRATCH_RSP)
 	movq	PCPU(RSP0),%rsp