linux: address executable stack warnings

Mark assembly files as not requiring executable stacks.

This still leaves linux32_vdso.so without a .note.GNU-stack section in
the gcc build for now.

Reviewed by:	imp, kib
Differential Revision:	https://reviews.freebsd.org/D56894

sys/amd64/linux/linux_locore.asm

@@ -85,3 +85,5 @@ END(linux_rt_sigcode)
 	.balign 4
 	.previous
 #endif
+
+	.section .note.GNU-stack,"",%progbits

sys/amd64/linux/linux_support.S

@@ -227,3 +227,5 @@ ENTRY(futex_xorl_smap)
 	movq	%rax,PCB_ONFAULT(%r8)
 	ret
 END(futex_xorl_smap)
+
+	.section .note.GNU-stack,"",%progbits

sys/amd64/linux32/linux32_locore.asm

@@ -128,3 +128,5 @@ END(__kernel_vsyscall)
 	.balign 4
 	.previous
 #endif
+
+	.section .note.GNU-stack,"",%progbits

sys/amd64/linux32/linux32_support.S

@@ -227,3 +227,5 @@ ENTRY(futex_xorl_smap)
 	movq	%rax,PCB_ONFAULT(%r8)
 	ret
 END(futex_xorl_smap)
+
+	.section .note.GNU-stack,"",%progbits

sys/arm64/linux/linux_locore.asm

@@ -51,3 +51,5 @@ __user_rt_sigreturn:
 	mov	x8, #LINUX_SYS_linux_rt_sigreturn
 	svc	#0
 EEND(__kernel_rt_sigreturn)
+
+	.section .note.GNU-stack,"",%progbits

sys/arm64/linux/linux_support.S

@@ -176,3 +176,5 @@ ENTRY(futex_xorl)
 END(futex_xorl)
 
 GNU_PROPERTY_AARCH64_FEATURE_1_NOTE(GNU_PROPERTY_AARCH64_FEATURE_1_VAL)
+
+	.section .note.GNU-stack,"",%progbits

sys/compat/linux/linux_vdso_inc.S

@@ -37,3 +37,5 @@ _binary_linux_vdso_so_o_end:
 #include <machine/asm.h>
 GNU_PROPERTY_AARCH64_FEATURE_1_NOTE(GNU_PROPERTY_AARCH64_FEATURE_1_VAL)
 #endif
+
+	.section .note.GNU-stack,"",%progbits

sys/i386/linux/linux_locore.asm

@@ -122,3 +122,5 @@ END(__kernel_vsyscall)
 	.balign 4
 	.previous
 #endif
+
+	.section .note.GNU-stack,"",%progbits