section_tags.h: cleanup

Some stuff that was platform-specific is made common, and some repeated
code was made a macro. __in_section() is used elsewhere in the codebase
and its 2nd and 3rd arguments are not necessarily filename/counter.

GCC-specific stuff moved to the toolchain header.

Change-Id: Ibfae919b6dd8a77210801c14e9a1128b43bd63f6
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>