Fix initialization on Frontends without variable support.

Fix initialization on Frontends without variable support.

Frontends that do not support RETRO_ENVIRONMENT_GET_VARIABLE (like
miniretro & nanoarch) cause a segfault in the CPU emulation path unless
the BIOS file is present. This is due to the emulator not supporting
running without a BIOS in dynarec mode and the code not being able to
swap CPU types (unless the callback returns true).