Add a wrapper for evaluating _OSC methods.

This wrapper does not translate errors in the first word to ACPI
error status returns. Use this wrapper in the acpi_cpu(4) driver in
place of the existing _OSC code. While here, fix a bug where the wrong
count of words was passed when invoking _OSC.

Reviewed by: jkim
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6022