x86: convert to use __HEAD and HEAD_TEXT macros.

This has the consequence of changing the section name use for head
code from ".text.head" to ".head.text".  It also eliminates the
".text.head" output section (instead placing head code at the start of
the .text output section), which should be harmless.

This patch only changes the sections in the actual kernel, not those
in the compressed boot loader.

Signed-off-by: Tim Abbott <tabbott@ksplice.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>