Move PLTs to the beginning of amd64 kernel modules.

As with .text, the aim is to ensure that executable sections are
segregated from the rest, to avoid creation of writeable and executable
mappings.  Recent versions of LLVM emit a PLT in firmware modules.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D26444

sys/conf/ldscript.kmod.amd64

@@ -34,6 +34,10 @@
 
 SECTIONS
 {
+	.plt		:
+	{
+		*(.plt)
+	}
 	.text		:
 	{
 		*(.text .text.*)