icache: Log 36 bits of instruction rather than 32

icache: Log 36 bits of instruction rather than 32

This expands the field in the log buffer that stores the instruction
fetched from the icache to 36 bits, so that we get the insn_code and
illegal instruction indication.  To do this, we reclaim 3 unused bits
from execute1's portion and one other unused bit (previously just set
to 0 in core.vhdl).

This also alters the trigger behaviour to stop after one quarter of
the log buffer has been filled with samples after the trigger, or 256
entries, whichever is less.  This is to ensure that the trigger event
doesn't get overwritten when the log buffer is small.

This updates fmt_log to the new log format.  Valid instructions are
printed as a decimal insn_code value followed by the bottom 26 bits of
the instruction.  Illegal instructions are printed as "ill" followed
by the full 32 bits of the instruction.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>