* Introduce 64-bit tick counter
* Let the timer always count when possible, also if it already fired
* Simplify the kernel syscall API to have one current time call,
which returns the elapsed microseconds since boot
* Acknowledge receive of page-fault signal with ack_signal,
but restart thread execution separately
* use kill_signal_context when disolving a pager_object to prevent race
* Remove bureaucracy in form of Thread_event and Signal_ack_handler
* remove dead code in riscv, namely Thread_base definition
* translation_table_insertions function for ARM drops out,
which was overcautious
Instead of holding SPEC-variable dependent files and directories inline
within the repository structure, move them into 'spec' subdirectories
at the corresponding levels, e.g.:
repos/base/include/spec
repos/base/mk/spec
repos/base/lib/mk/spec
repos/base/src/core/spec
...
Moreover, this commit removes the 'platform' directories. That term was
used in an overloaded sense. All SPEC-relative 'platform' directories are
now named 'spec'. Other files, like for instance those related to the
kernel/architecture specific startup library, where moved from 'platform'
directories to explicit, more meaningful places like e.g.: 'src/lib/startup'.
Fix#1673
