Turn off acpi_timer_test on !i386 by default

The ACPI timer test was introduced in 2002 to detect an erratum in
chipsets used with Pentium II and Pentium III processors. No other
hardware is known to be affected, so on non-i386 systems it should
be safe to skip the test.

Turning off this test speeds up the FreeBSD boot process by roughly
140 ms on an EC2 c5.xlarge instance.

The previous behaviour can be restored by setting
hw.acpi.timer_test_enabled=1
in /boot/loader.conf.

Requested by: jhb, imp
Sponsored by: https://www.patreon.com/cperciva