retguard for amd64 system calls

Todd Mortimer (mortimer@) has
committed
(to -current)
retguard
for amd64 system calls:

CVSROOT:	/cvs
Module name:	src
Changes by:	mortimer@cvs.openbsd.org	2023/01/10 18:55:18

Modified files:
	lib/libc/arch/amd64: SYS.h 
	lib/libc/arch/amd64/sys: Ovfork.S brk.S sbrk.S sigpending.S 
	                         sigprocmask.S sigsuspend.S 
	                         tfork_thread.S 
	libexec/ld.so/amd64: SYS.h 

Log message:
Add retguard to amd64 syscalls.

Since we got rid of padded syscalls we have enough registers to do this.

ok deraadt@ ok kettenis@

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