x86: Add external retpoline thunk

This allows use of -mindirect-branch=thunk-extern, and is only a few
bytes of dead code if not used.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D60524

sys/amd64/amd64/support.S

@@ -1996,4 +1996,14 @@ ENTRY(cpu_sync_core)
 	iretq
 END(cpu_sync_core)
 
+/* External Retpoline thunk */
+ENTRY(__x86_indirect_thunk_r11)
+	callq	2f
+1:	pause
+	lfence
+	jmp	1b
+2:	movq	%r11, (%rsp)
+	retq
+END(__x86_indirect_thunk_r11)
+
 	.section .note.GNU-stack,"",%progbits

sys/i386/i386/support.S

@@ -587,4 +587,14 @@ ENTRY(cpu_sync_core)
 	iretl
 END(cpu_sync_core)
 
+/* External retpoline thunk */
+ENTRY(__x86_indirect_thunk_eax)
+	calll	2f
+1:	pause
+	lfence
+	jmp	1b
+2:	movl	%eax, (%esp)
+	retl
+END(__x86_indirect_thunk_eax)
+
 	.section .note.GNU-stack,"",%progbits