systemc: Get sc_main to work even when dynamically linked.

systemc: Get sc_main to work even when dynamically linked.

Unfortunately multiply defined weak symbols don't work like they're
supposed to when the different versions are across dynamic linking
boundaries.

Fortunately, a weak symbol with no definition at all will still
consistently evaluate as 0, and a singularly defined weak symbol will
still resolve correctly.

Instead of relying on a weak version of sc_main being overridden by a
strong/non-default definition, this change leaves it as undefined and
detects at run time whether the symbol resolves to 0 or to an actual
code location.

Change-Id: I31c4ff769b0c52277c6cf4845ca3f85000009583
Reviewed-on: https://gem5-review.googlesource.com/11610
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>