Debug output: Do not emit function nesting level for kernel build.

The nesting level is really only useful during a single-thread execution.
Therefore, only enable this output for the AcpiExec utility.
Also, only emit the thread ID when executing under AcpiExec.
(Context switches are still detected and a message is emitted.)
ACPICA BZ 972.
Update ACPICA build instruction file (build.txt).

Clarify some steps.