arch_prctl used to be amd64 only. But since linux 4.12 it is also implemented or i386. And since glibc 2.28 (not yet released) it will be used to see if the cpu/kernel provides CET support.

See also https://bugs.kde.org/show_bug.cgi?id=396887

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 I just made it ENOSYS on Fedora:

diff --git a/coregrind/m_syswrap/syswrap-x86-linux.c b/coregrind/m_syswrap/syswrap-x86-linux.c
index bec145a..11bb13d 100644
--- a/coregrind/m_syswrap/syswrap-x86-linux.c
+++ b/coregrind/m_syswrap/syswrap-x86-linux.c
@@ -1604,7 +1604,10 @@ static SyscallTableEntry syscall_table[] = {
    LINX_(__NR_sendmsg,           sys_sendmsg),          // 370
    LINXY(__NR_recvfrom,          sys_recvfrom),         // 371
    LINXY(__NR_recvmsg,           sys_recvmsg),          // 372
-   LINX_(__NR_shutdown,          sys_shutdown)          // 373
+   LINX_(__NR_shutdown,          sys_shutdown),         // 373
+
+   /* Explicitly not supported on i386 yet. */
+   GENX_(__NR_arch_prctl,        sys_ni_syscall)        // 384
 };
 
 SyscallTableEntry* ML_(get_linux_syscall_entry) ( UInt sysno )

This is fine for glibc/ld.so usage since any failure indicates no CET support.