Bug 397012 - glibc ld.so uses arch_prctl on i386.

Bug 397012 - glibc ld.so uses arch_prctl on i386.

arch_prctl used to be amd64 only. But since linux 4.12 it is also
implemented for i386. And since glibc 2.28 ld.so will use it to see
if the cpu/kernel provides CET support.

To prevent seeing:

WARNING: unhandled x86-linux syscall: 384
You may be able to write your own handler.
Read the file README_MISSING_SYSCALL_OR_IOCTL.
Nevertheless we consider this a bug.  Please report
it at http://valgrind.org/support/bug_reports.html.

on every program run under valgrind just make it explicitly
not implemented (ENOSYS). This is fine for the glibc usage.
It just indicates there is no CET support.