python: Add a generalized mechanism to configure stats

Add a mechanism to configure the stat output format using a URL-like
syntax. This makes it possible to specify both an output format
(currently, only text is supported) and override default
parameters.

On the Python-side, this is implemented using a helper function
(m5.stats.addStatVisitor) that adds a visitor to the list of active
stat visitors. The helper function parses a URL-like stat
specification to determine the stat output type. Optional parameters
can be specified to change how stat visitors behave.

For example, to output stats in text format without stat descriptions:

    m5.stats.addStatVisitor("text://stats.txt?desc=False")

From the command line:

    gem5.opt --stats-file="text://stats.txt?desc=False"

Internally, the stat framework uses the _url_factory decorator
to wrap a Python function with the fn(path, **kwargs) signature in a
function that takes a parsed URL as its only argument. The path and
keyword arguments are automatically derived from the URL in the
wrapper function.

New output formats can be registered in the m5.stats.factories
dictionary. This dictionary contains a mapping between format names
(URL schemes) and factory methods.

To retain backwards compatibility, the code automatically assumes that
the user wants text output if no format has been specified (i.e., when
specifying a plain path).

Change-Id: Ic4dce93ab4ead07ffdf71e55a22ba0ae5a143061
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Ilias Vougioukas <ilias.vougioukas@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>

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