Fix marking threadasm.S as not requiring an executable stack.

- Use of @progbits breaks on ARM, use %progbits instead.
- Works on FreeBSD, too.
- Works only for ELF files.

See https://wiki.gentoo.org/wiki/Project:Hardened/GNU_stack_quickstart for more info.
Merge pull request #885 from redstar/execstack

Fix marking threadasm.S as not requiring an executable stack.