tell the assembler to mark all files as not requiring executable stack

for some reason this option is undocumented. not sure when it was
added, so I'm using a configure test. gcc was already setting the mark
correctly for C files, but assembler source files would need ugly
.note boilerplate in every single file to achieve this without the
option to the assembler.

blame whoever thought it would be a good idea to make the stack
executable by default rather than doing it the other way around...