The kernel-agnostic 'Trace::timestamp' function for arm_64 executes the
'mrs %0, pmccntr_el0' instruction, which is not permitted for user-level
programs on Linux. This patch shadows the generic timestamp.h header
with dummy that returns zero. This return value prompts the timeout
framework to disable the interpolation of time based on timestamps. This
avoid the illegal-instruction abort but comes with two llimitations:
First, time measurements are effectlively limited to a granulariry of 1
millisecond (deliberately constrained by the timer driver).
The quirk is applied when using the base-linux API. Should a generic
application (that uses the base API only) call 'Trace::timestamp'
directly, the illegal instruction is executed.
Issue #4136
Since the timer and timeout handling is part of the base library (the
dynamic linker), it belongs to the base repository.
Besides moving the timer and its related infrastructure (alarm, timeout
libs, tests) to the base repository, this patch also moves the timer
from the 'drivers' subdirectory directly to 'src' and disamibuates the
timer's build locations for the various kernels. Otherwise the different
timer implementations could interfere with each other when using one
build directory with multiple kernels.
Note that this patch changes the include paths for the former os/timer,
os/alarm.h, os/duration.h, and os/timed_semaphore.h to base/.
Issue #3101
