Hide acpi_timer_test behind a tunable

[cperciva]

Summary

When hw.acpi.timer_test_enabled is set to 0, this makes acpi_timer_test return 1
without actually testing the ACPI timer; this results in ACPI-fast being used rather
than potentially using ACPI-safe.

The ACPI timer testing was introduced in 2002 as a workaround for errata in
Pentium II and Pentium III chipsets, and is unlikely to be needed in 2021.

While I'm here, add TSENTER/TSEXIT to make it easier to see the time spent
on the test (if it is enabled).

[cperciva]

Assuming this is approved, I plan to
(a) MFC it as-is to stable/13,
(b) Change the default in HEAD to acpi_timer_test_enabled = 0, and
(c) Nuke the test entirely in 15-CURRENT if nobody squawks while running 14.x.

[allanjude]

Reviewed By: allanjude

[imp]

sys/dev/acpica/acpi_timer.c

82

Why not #ifdef this to be 1 if building on i386 only?

[cperciva]

sys/dev/acpica/acpi_timer.c

82

Just in case the workaround for broken Pentium II/III chipsets was also hiding issues with other more modern systems. I'm going to set it to 0 on HEAD anyway; I only have it set to 1 here because that's what I'll be MFCing (to avoid changing the default behaviour on -STABLE).

[cperciva]

Whoops, forgot to put a link to this in the commit message.