x86: Use a simple jump into long mode

x86: Use a simple jump into long mode

With the 64-bit descriptor we can use a jump instruction, rather than
pushing things on the stack.

Since the processor is in 64-bit mode by this point, pop a 64-bit value
from the stack, containing the target address.

This simplifies the code slightly, in particular its use of the stack.

Signed-off-by: Simon Glass <sjg@chromium.org>