x86/boot: Annotate the multiboot headers with size and type information

x86/boot: Annotate the multiboot headers with size and type information

This causes objdump not to try and disassemble the data.

While altering this area, switch to using .balign, and fill with 0xc2 to help
highlight the embedded padding (rather than having it filled with 0f 1f 40 00
which is a long nop).  Also, shorten the labels by stripping off the _start
suffix.

The end result is now:
  ffff82d080200000 <_start>:
  ffff82d080200000:       e9 af c1 1c 00          jmpq   ffff82d0803cc1b4 <__start>
  ffff82d080200005:       0f 1f 00                nopl   (%rax)

  ffff82d080200008 <multiboot1_header>:
  ffff82d080200008:       02 b0 ad 1b 03 00 00 00 fb 4f 52 e4 c2 c2 c2 c2     .........OR.....

  ffff82d080200018 <multiboot2_header>:
  ffff82d080200018:       d6 50 52 e8 00 00 00 00 88 00 00 00 a2 ae ad 17     .PR.............
  ffff82d080200028:       01 00 00 00 10 00 00 00 04 00 00 00 06 00 00 00     ................
  ffff82d080200038:       06 00 00 00 08 00 00 00 0a 00 01 00 18 00 00 00     ................
  ffff82d080200048:       00 00 20 00 ff ff ff ff 00 00 20 00 02 00 00 00     .. ....... .....
  ffff82d080200058:       04 00 01 00 0c 00 00 00 02 00 00 00 c2 c2 c2 c2     ................
  ffff82d080200068:       05 00 01 00 14 00 00 00 00 00 00 00 00 00 00 00     ................
  ffff82d080200078:       00 00 00 00 c2 c2 c2 c2 07 00 01 00 08 00 00 00     ................
  ffff82d080200088:       09 00 01 00 0c 00 00 00 5e c0 3c 00 c2 c2 c2 c2     ........^.<.....
  ffff82d080200098:       00 00 00 00 08 00 00 00                             ........

  ffff82d0802000a0 <__high_start>:
  ffff82d0802000a0:       0f 01 15 5f 8f 25 00    lgdt   0x258f5f(%rip)        # ffff82d080459006 <gdt_descr>

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