add crt0.o to BIOS target list

[hhe07]

Without the proposed change, crt0.o is marked by Make as an intermediate file and removed:

Must remake target 'all'.
python3 -m litex.soc.software.memusage bios.elf {file path}/litex/mor1kx_standard_32_F
alse/software/bios/../include/generated/regions.ld or1k-elf
Putting child 0x55a2098374e0 (all) PID 41438 on the chain.
Live child 0x55a2098374e0 (all) PID 41438

ROM usage: 36.54KiB     (57.09%)
RAM usage: 1.74KiB      (21.78%)

Reaping winning child 0x55a2098374e0 PID 41438
Removing child 0x55a2098374e0 PID 41438 from chain.
Successfully remade target file 'all'.
Removing intermediate files...
rm crt0.o

crt0.o is potentially valuable for building software for a LiteX sim target (I'm currently working on a script to run Embench on a simulated target), so it would be nice to keep it around. It's possible to use a dedicated Makefile to create crt0.o, but doing so unnecessarily re-compiles a file which LiteX compiles anyway.