gdb: add scoped_time_it

gdb: add scoped_time_it

New in v2:

 - actually use m_enabled in the constructor and destructor
 - output using gdb_stdlog->write_async_safe instead of gdb_printf

scoped_time_it is a small utility that measures and prints how much time
a given thread spent in a given scope.  Similar to the time(1) command,
it prints the time spent in user mode, system mode, and the wall clock
time.  It also prints the CPU utilization percentage, which is:

  (user + sys) / wall

This can help spot cases where the workload is not well balanced between
workers, or the CPU utilization is not optimal (perhaps due to
contention around a lock for example).

To use it, just add it in some scope.  For instance, a subsequent patch
adds it here:

      workers.add_task ([this, task_count, iter, last] ()
	{
	  scoped_time_it time_it ("DWARF indexing worker");
	  process_cus (task_count, iter, last);
	});

On destruction, if enabled, it prints a line showing the time spent by
that thread, similar to what time(1) prints.

The example above prints this (one line for each worker thread):

    Time for "DWARF indexing worker": wall 0.173, user 0.120, sys 0.034, user+sys 0.154, 89.0 % CPU
    Time for "DWARF indexing worker": wall 0.211, user 0.144, sys 0.047, user+sys 0.191, 90.5 % CPU
    Time for "DWARF indexing worker": wall 0.368, user 0.295, sys 0.057, user+sys 0.352, 95.7 % CPU
    Time for "DWARF indexing worker": wall 0.445, user 0.361, sys 0.072, user+sys 0.433, 97.3 % CPU
    Time for "DWARF indexing worker": wall 0.592, user 0.459, sys 0.113, user+sys 0.572, 96.6 % CPU
    Time for "DWARF indexing worker": wall 0.739, user 0.608, sys 0.115, user+sys 0.723, 97.8 % CPU
    Time for "DWARF indexing worker": wall 0.831, user 0.677, sys 0.140, user+sys 0.817, 98.3 % CPU
    Time for "DWARF indexing worker": wall 0.949, user 0.789, sys 0.144, user+sys 0.933, 98.3 % CPU

The object is only enabled if per_command_time (controlled by "maint set
per-command time") is true at construction time.  I wanted to avoid
adding a new command for now, but eventually if there are too many
scoped_time_it around the code base and we want to be able to enabled
them selectively (e.g. just the ones in the DWARF reader, or in the
symbol searching functions, etc), we could have a dedicated command for
that.

I added this functionality to GDB because it relies on gdb_printf and
per_command_time, but if we ever need it in gdbsupport, I'm sure we
could find a way to put it there.

Change-Id: I5416ac1448f960f44d85f8449943d994198a271e
Approved-By: Tom Tromey <tom@tromey.com>