unistd: add __NR_prlimit64 syscall numbers

Add __NR_prlimit64 syscall numbers to asm-generic. Add them also to
asm-x86, both 32 and 64-bit.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>