sim: testsuite: setup per-port toolchain settings for multitarget build

Gas does not support multitarget builds -- it still only supports
a single input & output format.  ld is a bit better, but requires
manual flags to select the right output.  This makes it impossible
to run the complete testsuite in a multitarget build.

To address this limitation, create a suite of FOR_TARGET variables
so these can be set to precompiled as & ld programs.  It requires
a bit of setup ahead of time, but it's a one-time cost, and makes
running the full testsuite at once much easier.