Use ACPI SPCR on x86

This takes the SPCR code currently in uart_cpu_arm64.c, moves it into
a new uart_cpu_acpi.c (with some associated refactoring), and uses it
from both arm64 and x86.

An SPCR serial port address AccessWidth field value of 0 ("reserved")
is now treated as 1 ("byte access") in order to work around a buggy
SPCR table on Amazon EC2 i3.metal instances.

Reviewed by: manu, Greg V
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D20357