arm: introduce ret_from_kernel_execve(), switch to generic kernel_execve()

arm: introduce ret_from_kernel_execve(), switch to generic kernel_execve()

ret_from_kernel_execve() is the asm tail of kernel_execve(), essentially;
the rest became shiny new generic implementation, right in fs/exec.c.
An architecture can provide ret_from_kernel_execve() and define
__ARCH_WANT_KERNEL_EXECVE; then it'll get the generic one.

Another new helper: current_pt_regs().  Default is task_pt_regs(current),
and it's perfectly fine to just leave it as is.  However, an architecture
might choose to provide an optimized variant in its thread_info.h

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>