Starting with binutils 2.39:

The ELF linker will now generate a warning message if the stack is made executable. Similarly it will warn if the output binary contains a segment with all three of the read, write and execute permission bits set. These warnings are intended to help developers identify programs which might be vulnerable to attack via these executable memory regions.

Normally, the stack is marked as non-executable by the C compiler, but when you are using raw assembly it gets marked as executable, the assumption being that assembly authors want an executable stack. Adding the .note.GNU-stack section at the bottom of the .s files tells the linker the stack shouldn't be executable.

Updated header copyrights in assembly source files in addition