Remove cwd related syscalls and SYS_CHMOD.

See merge request !71

Remove old exec/clone system calls in favor of userspace_fexec

See merge request !70

First, auxiliary vectors are no longer left here in syscall, but remain
in relibc. The rationale behind removing them was that they are no
longer used by the kernel in any way, as exec is moved to userspace.

Second, a ptrace event for address space switches was added. This is to
allow tracers to reopen their mem file when that mem file points to
different memory (otherwise mem can be decoupled from address spaces and
simply use the one that the context uses at the time).

Update toolchain, removing dependence on nightly features

See merge request !68

This is to hopefully avoid having to update syscall everywhere again,
when fixing aarch64.

Add fsbase/gsbase reg struct and more ELF auxiliary vectors.

See merge request !67

Add FUTEX_WAIT64.

See merge request !66

Specify feature(const_fn_trait_bound)

See merge request !65