kboot: .note.GNU-stack is needed

Add '.section .note.GNU-stack,"",%progbits' to all assembler. Newer
versions of clang complain when this isn't present because executable
stacks are going away in the future. We don't need an executable stack
anyway.

Sponsored by:		Netflix
Reviewed by:		kevans, andrew, emaste, jhibbits
Differential Revision:	https://reviews.freebsd.org/D49855

stand/kboot/kboot/arch/aarch64/tramp.S

@@ -99,3 +99,5 @@ tramp_size:
 	.globl	tramp_data_offset
 tramp_data_offset:
 	.long	trampoline_data-tramp
+
+	.section .note.GNU-stack,"",%progbits

stand/kboot/kboot/arch/amd64/amd64_tramp.S

@@ -105,3 +105,5 @@ tramp_size:
 tramp_data_offset:
 	.long	trampoline_data-tramp
 	.size	tramp_data_offset, 4
+
+	.section .note.GNU-stack,"",%progbits

stand/kboot/kboot/arch/powerpc64/kerneltramp.S

@@ -99,3 +99,5 @@ endkerneltramp:
 	.data
 CNAME(szkerneltramp):
 	.long endkerneltramp - CNAME(kerneltramp)
+
+	.section .note.GNU-stack,"",%progbits

stand/kboot/libkboot/arch/aarch64/host_syscall.S

@@ -16,3 +16,5 @@ ENTRY(host_syscall)
 	ret
 /* Note: We're exposing the raw return value to the caller */
 END(host_syscall)
+
+	.section .note.GNU-stack,"",%progbits

stand/kboot/libkboot/arch/amd64/host_syscall.S

@@ -27,3 +27,5 @@ ENTRY(host_syscall)
 	ret
 /* Note: We're exposing the raw return value to the caller */
 END(host_syscall)
+
+	.section .note.GNU-stack,"",%progbits

stand/kboot/libkboot/arch/powerpc64/host_syscall.S

@@ -30,3 +30,5 @@ ENTRY(host_syscall)
 	blr
 /* Note: We're exposing the raw return value to the caller */	
 END(host_syscall)
+
+	.section .note.GNU-stack,"",%progbits