systemc: Add a non-standard sc_time constructor and from_string.

The sc_time constructor was being called, but because of implicit type
conversions, a const char * was being treated as a bool and totally
unrelated constructor was being called.

This change adds and implements the missing but non-standard
constructor. It also implements the from_string function which uses
that constructor.

Change-Id: I21e7e40fd1a8d1c579b1abdc2036d016501f510c
Reviewed-on: https://gem5-review.googlesource.com/c/13191
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement the sc_time_tuple class.

This class is non-standard and is an implementation detail in
Accellera's implementation, but is referred to directly by the tests.
It does the same thing as the time printing function, so rather than
having duplicate code the printing function now uses the sc_time_tuple
class even though it was doing fine on its own already.

Change-Id: I69594ed0651f212ded6d979d60523bb3b0a789b1
Reviewed-on: https://gem5-review.googlesource.com/c/13189
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement some of the basics of sc_time.

This is a very incomplete implementation, but is enough to unblock
implementing some other dependent features.

Change-Id: Ibd2f3476fe01389e277c6956d48337f551d14acd
Reviewed-on: https://gem5-review.googlesource.com/12034
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add the non-standard sc_time_tuple class.

Change-Id: Ia3d6a6a4ea3383c82605653faac570ced7bebb70
Reviewed-on: https://gem5-review.googlesource.com/11277
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add deprecated sc_time constructors.

Change-Id: Iffae751272302ff2996258a1ab31b086e12bbb8d
Reviewed-on: https://gem5-review.googlesource.com/11275
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add some deprecated sc_time factory methods.

Change-Id: I0d9a7040a48b9f0d0079e9daecaf44ea78c186de
Reviewed-on: https://gem5-review.googlesource.com/11273
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Add deprecated default time unit management functions.

Having default time units is deprecated, and so are the functions that
manage them.

Change-Id: Ie21f9a5fca9868dd4f0adcd9f32c568fbec1fa72
Reviewed-on: https://gem5-review.googlesource.com/11270
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Seperate the "external" header interface.

Most (but not all) of the SystemC headers are part of the "external"
interface that an existing, standard compliant module would include
through <systemc.h> or <systemc>. Since those follow slightly different
rules (relative includes, no gem5 includes), this change separates them
out so that they're easier to identify.

Also, this change moves the other files into a "core" subdirectory,
with the intention to add a "dt", aka data type, directory some time in
the future when those standard defined types are implemented.

Change-Id: Ida63f9cc0bc0431024d4dd691cc5b22b944a99a8
Reviewed-on: https://gem5-review.googlesource.com/10835
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>