Disable executable stacks on assembly objects (#2071)

Add `--noexecstack` to the assembler flags so the resulting binary
will link with a non-executable stack.

Fixes zerotier/ZeroTierOne#1179

Co-authored-by: Joseph Henry <[email protected]>

make-linux.mk

@@ -357,6 +357,9 @@ endif
 override CFLAGS+=-fPIC -fPIE
 override CXXFLAGS+=-fPIC -fPIE
 
+# Non-executable stack
+override ASFLAGS+=--noexecstack
+
 .PHONY: all
 all:	one