Makefile.mk: Mark stack as not executable

Suppress the warning:
    missing .note.GNU-stack section implies executable stack
    NOTE: This behaviour is deprecated and will be removed in a
    future version of the linker

Since we don't need an executable stack this is fine. Some newer
linkers like LLD even default to this.

Change-Id: Ib787cc464e0924ab57575cec9fbfd1d59bdd3481
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83560
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Nico Huber <nico.h@gmx.de>

Makefile.mk

@@ -603,6 +603,7 @@ LDFLAGS_common := --gc-sections
 LDFLAGS_common += -nostdlib
 LDFLAGS_common += --nmagic
 LDFLAGS_common += -static
+LDFLAGS_common += -z noexecstack
 # Disable warning on segments with RWX.
 # All loadable sections are placed in the same segment for simplicity.
 LDFLAGS_common += --no-warn-rwx-segments