amd64: prevents speculations over swapgs reload of %gs base.

amd64: prevents speculations over swapgs reload of %gs base.

Such speculations could use user-controlled %gs base, esp. since
FreeBSD supports WRGSBASE instructions.

Place LFENCEs on entry for each basic block after the test for
previous kernel/user mode on the kernel entry, which prevents the
speculation.  Code accesses %gs-based PCPU before any serialization
instructions are executed, like %cr3 reload for KPTI.

With pti disabled, on haswell i7-4770S machine, "syscall_timings getppid"
shows when no lfence is added to syscall path:
test	loop	time	iterations	periteration
getppid	0	1.040918865	4643611	0.000000224
getppid	1	1.004985962	4481816	0.000000224
getppid	2	1.005196483	4482363	0.000000224
with lfence:
getppid	0	1.043701091	4554779	0.000000229
getppid	1	1.016930328	4438094	0.000000229
getppid	2	1.023223117	4466640	0.000000229
and ministat reports 'No difference proven at 95.0% confidence.'

Security:	CVE-2019-1125
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week