x86: misc boot/link tweaking

x86: misc boot/link tweaking

 * Introduce symbols bounding the multiboot1 header, which helps clarify that
   it is data and not code corruption when viewing the disassembly.
 * Move the __high_start symbol to its implementation, and declare it
   correctly as ENTRY()
 * Move the l1_identmap construction to be with all the other pagetables, and
   within __page_tables_{start,end}.  This won't affect the EFI relocation
   algorithm, as l1_identmap contains no relocations.
 * Move the cpu0_stack alignment check to the linker.  Chances are very good
   that a binary with a misaligned stack won't get as far as the test.
 * Use MB() in linker script.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>