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

[brooks]

Summary

Explicitly disable executable stacks in the syscall stubs on all
architectures. Previously, aarch64 and riscv64 didn't include the
.note.GNU-stack note due to an apparent oversight and it being opt-in.

This appears to have been harmless in practice, but better to be clear
in case a different compiler/linker has different defaults.

Reported by: jrtc27

[kib]

Does it matter/used on arm64 and risc-v? I remember that psABIs require nx stack always for them. It might be that some (old ?) linker complained about the note on these arches.

[jrtc27]

In D44883#1023197, @kib wrote:

Does it matter/used on arm64 and risc-v? I remember that psABIs require nx stack always for them. It might be that some (old ?) linker complained about the note on these arches.

GCC nested functions require an executable stack, but I believe the default is NX?