sim: mips: always resolve active bfd mach dynamically

sim: mips: always resolve active bfd mach dynamically

Don't assume that the default bfd that we configured for is the one
that is always active when running a program.  We already have access
to the real runtime value, so use it directly.  This simplifies the
code quite a bit, and will make it easier to support multiple mach's
in a single binary.