[IA64] Use .ref.text, not .text.init for start_ap.

It seems that start_ap doesn't need to be in a special location in the
kernel, but it references some init code so it should be in .ref.text.

Since this is the only thing in the .text.head section, eliminate
.text.head from the linker script.

Signed-off-by: Tim Abbott <tabbott@ksplice.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>