x86: Remove vsyscall usage

x86: Remove vsyscall usage

This patch removes the vsyscall usage for x86_64 port.  As indicated
by kernel code comments [1], vsyscalls are a legacy ABI and its concept
is problematic:

- It interferes with ASLR.
- It's awkward to write code that lives in kernel addresses but is
  callable by userspace at fixed addresses.
- The whole concept is impossible for 32-bit compat userspace.
- UML cannot easily virtualize a vsyscall.

The VDSO is a better approach for such functionality.  Tested on i686,
x86_64, and x32.

	* sysdeps/unix/sysv/linux/i386/gettimeofday.c
	(__gettimeofday_syscall): Remove vsyscall fallback.
	* sysdeps/unix/sysv/linux/i386/time.c (__time_syscall): Likewise.
	* sysdeps/unix/sysv/linux/x86/gettimeofday.c (__gettimeofday_syscall):
	Add syscall fallback function.
	(gettimeofday_ifunc): Use __gettimeofday_syscall as fallback mechanism
	if vDSO is not present.
	* sysdeps/unix/sysv/linux/x86/time.c (__time_syscall): Add syscall
	fallback function.
	(time_ifunc): Use __time_syscall as fallback mechanism if vDSO is not
	present.
	* sysdeps/unix/sysv/linux/x86_64/gettimeofday.c: Remove file.
	* sysdeps/unix/sysv/linux/x86_64/time.c: Likewise.

[1] arch/x86/kernel/vsyscall_64.c