x86/CET: Extend arch_prctl syscall for CET control

x86/CET: Extend arch_prctl syscall for CET control

CET arch_prctl bits should be defined in <asm/prctl.h> from Linux kernel
header files.  Add x86 <include/asm/prctl.h> for pre-CET kernel header
files.

Note: sysdeps/unix/sysv/linux/x86/include/asm/prctl.h should be removed
if <asm/prctl.h> from the required kernel header files contains CET
arch_prctl bits.

 /* CET features:
    IBT:   GNU_PROPERTY_X86_FEATURE_1_IBT
    SHSTK: GNU_PROPERTY_X86_FEATURE_1_SHSTK
  */

 /* Return CET features in unsigned long long *addr:
      features: addr[0].
      shadow stack base address: addr[1].
      shadow stack size: addr[2].
  */
 # define ARCH_CET_STATUS		0x3001
 /* Disable CET features in unsigned int features.  */
 # define ARCH_CET_DISABLE		0x3002
 /* Lock all CET features.  */
 # define ARCH_CET_LOCK			0x3003
 /* Allocate a new shadow stack with unsigned long long *addr:
      IN: requested shadow stack size: *addr.
      OUT: allocated shadow stack address: *addr.
  */
 # define ARCH_CET_ALLOC_SHSTK		0x3004
 /* Return legacy region bitmap info in unsigned long long *addr:
     address: addr[0].
     size: addr[1].
  */
 # define ARCH_CET_LEGACY_BITMAP	0x3005

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
	* sysdeps/unix/sysv/linux/x86/include/asm/prctl.h: New file.
	* sysdeps/unix/sysv/linux/x86/cpu-features.c: Include
	<sys/prctl.h> and <asm/prctl.h>.
	(get_cet_status): Call arch_prctl with ARCH_CET_STATUS.
	* sysdeps/unix/sysv/linux/x86/dl-cet.h: Include <sys/prctl.h>
	and <asm/prctl.h>.
	(dl_cet_allocate_legacy_bitmap): Call arch_prctl with
	ARCH_CET_LEGACY_BITMAP.
	(dl_cet_disable_cet): Call arch_prctl with ARCH_CET_DISABLE.
	(dl_cet_lock_cet): Call arch_prctl with ARCH_CET_LOCK.
	* sysdeps/x86/libc-start.c: Include <startup.h>.