lib{c,sys}: .note.GNU-stack in syscall stubs

Explicitly disable executable stacks in the syscall stubs on all
architectures. Previously, aarch64 and riscv64 didn't include the
.note.GNU-stack note due it being disabled by default in those ABIs.

This appears to have been harmless in practice, but better to be clear
in case a different compiler/linker has different defaults. This also
reduces special cases in the Makefile.

Reported by: jrtc27
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44883