x86/boot: fix BIOS memory corruption on certain IBM systems

x86/boot: fix BIOS memory corruption on certain IBM systems

IBM System x3530 M4 BIOSes (including the latest available at the time of this
patch) will corrupt a byte at physical address 0x105ff1 to the value of 0x86
if %esp has the value 0x00080000 when issuing an `int $0x15 (ax=0xec00)` to
inform the system about our intended operating mode.

Xen gets unhappy when the bootloader has placed it's .text section in over
this specific region of RAM.

After dropping into 16bit mode, clear all 32 bits of %esp, and for the BIOS
call already documented to be affected by BIOS bugs clear all GPRs.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Release-acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>