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...

configure

@@ -247,6 +247,13 @@ fi
 tryflag CFLAGS_AUTO -fno-unwind-tables
 tryflag CFLAGS_AUTO -fno-asynchronous-unwind-tables
 
+#
+# The GNU toolchain defaults to assuming unmarked files need an
+# executable stack, potentially exposing vulnerabilities in programs
+# linked with such object files. Fix this.
+#
+tryflag CFLAGS_AUTO -Wa,--noexecstack
+
 #
 # Some optimization levels add bloated alignment that hurt performance
 #