The default executable stack setting on Linux can be fixed in two different ways:
- By adding the `.section .note.GNU-stack,"",%progbits` special incantation
- By passing the `--noexecstack` flag to the assembler
This patch implements both, but only one of them is strictly necessary.
I've also added some additional hardening flags to the Makefile. May not be portable.
Whereas vinserti64x4 is present on AVX512F, vinserti32x8 requires
AVX512DQ, which we do not test for. At this point there is not
much risk of incompatibility, since Skylake-X chips have all the
requires instruction sets, but let's be precise about this.
