xen/arm: Fix UBSAN failure in start_xen()

xen/arm: Fix UBSAN failure in start_xen()

When running Xen on arm32, in scenario where Xen is loaded at an address
such as boot_phys_offset >= 2GB, UBSAN reports the following:

(XEN) UBSAN: Undefined behaviour in arch/arm/setup.c:739:58
(XEN) pointer operation underflowed 00200000 to 86800000
(XEN) Xen WARN at common/ubsan/ubsan.c:172
(XEN) ----[ Xen-4.19-unstable  arm32  debug=y ubsan=y  Not tainted ]----
...
(XEN) Xen call trace:
(XEN)    [<0031b4c0>] ubsan.c#ubsan_epilogue+0x18/0xf0 (PC)
(XEN)    [<0031d134>] __ubsan_handle_pointer_overflow+0xb8/0xd4 (LR)
(XEN)    [<0031d134>] __ubsan_handle_pointer_overflow+0xb8/0xd4
(XEN)    [<004d15a8>] start_xen+0xe0/0xbe0
(XEN)    [<0020007c>] head.o#primary_switched+0x4/0x30

The failure is reported for the following line:
(paddr_t)(uintptr_t)(_start + boot_phys_offset)

This occurs because the compiler treats (ptr + size) with size bigger than
PTRDIFF_MAX as undefined behavior. To address this, switch to macro
virt_to_maddr(), given the future plans to eliminate boot_phys_offset.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Tested-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit e11f5766503c0ff074b4e0f888bbfc931518a169)