Revert "sim: mn10300: drop unused sim-main.c"

This reverts commit 681a422b855e4b20086554b170dae051361f00c7.

I missed that this was included via common/sim-inline.c.  I thought
I had grepped the top of the tree, but I must have only done mn10300.

Add a comment to make it clear where/how this file is used.