core/cortex-m*/task: Record 32-bit exception times

"time in exceptions" looks unreasonable after > 4294s (when 32-bit
microsecond timer wraps around).

This is because core/cortex-m/task.c:svc_handler records time
exc_start_time just before the interrupt handler for 32-bit
timer interrupt runs, so the high word of the system clock
(clksrc_high) is not updated yet (while the low 32-bits already
wrapped around). After the handler runs, clksrc_high is updated,
so there appear to be a 4294s gap between the 2 measurements.

Fix this by recording the low 32-bit timer value only. There will
never be more than 4294s between exceptions, and this fixes the
wrap-around issue as well.

BRANCH=none
BUG=chromium:914208
TEST=Flash kukui (cortex-m0) and kukui_scp (cortex-m), let system
     run for 4300+s, no more accounting error in "Time in exceptions".

Change-Id: If52855ef093ac1a1d38432555694c83742feb8f1
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1372876

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