base-stats: Add Units to Stats

base-stats: Add Units to Stats

This commit adds an option to add SI and its derived units to stats.

Units are now the third parameter of every Stat class constructor.

The following are convenient macros that could be used to add units
to stats,
   * UNIT_CYCLE: represents clock cycles.
   * UNIT_TICK: represents the count of gem5's Tick.
   * UNIT_SECOND: represents the base unit of time defined by SI.
   * UNIT_BIT: represents the number of computer bits.
   * UNIT_BYTE: represents 8 bits.
   * UNIT_VOLT: a SI derived unit measuring potential difference.
   * UNIT_JOULE: represents joule, a unit of energy, as defined by SI.
   * UNIT_WATT: represents 1 watt, where 1 watt = 1 joule / second.
   * UNIT_CELSIUS: represents 1 Celsius degree as defined by SI.
   * UNIT_RATE(T1, T2): represents the unit of a quantity of T1 divided
                        by a quantity of T2.
   * UNIT_RATIO: represents the unit of a quantity of unit T divided by
                 a quantity of unit T.
   * UNIT_UNSPECIFIED: the unit of the stat is unspecified. This type of
                       unit is mainly for backward compatibility. Newly
                       introduced stats should have the units specified.

This commit also alters the behavior of the ADD_STAT macro.
Specifically, ADD_STAT syntax is unchanged, but the unit of the stat
is hardcoded to be UNIT_UNSPECIFIED.

link: https://gem5.atlassian.net/browse/GEM5-849
This change is an effort towards supporting new stats schema:
https://github.com/gem5/stats-schema

Change-Id: I791704a6c4d9e06332797dbfc5eb611cb43f4710
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38855
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>