x86/xen: introduce non-hypercall based emergency print

The current xc_printf() function uses an hypercall in order to send character
buffers to the hypervisor for it to print on the hypervisor console (if the
hypervisor is configured to print such messages).

This requires the hypercall page to be initialized, which is extra work and can
go wrong.

On x86 instead of using the console IO hypercall use the debug console IO port,
also called "port E9 hack". This allows sending characters to Xen using an
outb instruction, without any initialization required.

Keep the previous hypervisor based implementation by using the weak attribute,
which allows each architecture to provide an alternate (arch-specific)
implementation.

Sponsored by: Cloud Software Group
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D43929