Add REGISTERS_CLOBBERED_BY_SYSCALL for x86-64

X86-64 system calls use a different calling convention, which clobbers
CC, %r11 an %rcx registers.  Define REGISTERS_CLOBBERED_BY_SYSCALL for
x86-64 inline asm statements.

	* sysdeps/unix/sysv/linux/x86_64/sysdep.h
	(REGISTERS_CLOBBERED_BY_SYSCALL): New.
	(INTERNAL_SYSCALL_NCS): Use it.
	(INTERNAL_SYSCALL_NCS_TYPES): Likewise.