loader: fix multiboot loading on UEFI

[royger]

Summary

The Xen kernel has no symbols tables, so calling lookup_symbol against
it triggers the following Divide by Zero fault:

Loading Xen kernel...
/boot/xen data=0x2809c8+0x149638 |
!! X64 Exception Type - 00(#DE - Divide Error) CPU Apic ID - 00000000 !!

Fix lookup_symbol to prevent the #DE fault from happening if the
symbol table is not loaded and also fix loadfile_raw to mark multiboot
kernels as relocatable, since the only multiboot kernel supported is
Xen and was already unconditionally booted as relocatable.

Fixes: f75caed644a5 ('amd64 UEFI loader: stop copying staging area to 2M physical')

[kib]

Note that 'relocatable' means variyng not VA, but PA. Loader still maps the kernel at KERNBASE.

[royger]

In D31507#710357, @kib wrote:

Note that 'relocatable' means variyng not VA, but PA. Loader still maps the kernel at KERNBASE.

Right. In the Xen case the kernel is started in protected mode with paging disabled, so it's only relocatable in the PA space, because there's no page table provided at entry point.

Thanks for the reviews.