IPL3: allow to disable RDRAM clearing

[rasky]

There are a very few, rare, legitimate use cases for not clearing the RAM at boot:

• somebody worked on a ROM to implement the "stop'n'swop" mechanism of Banjo Kazooie
• in game jam 25, a game was designed around resetting the console and keeping data floating in RAM

For these, people had to build their own patched IPL3. We could instead allow to disable RAM clearing via an opt-in flag in the header.

[rasky]

Notice that if RDRAM clearing is disabled, we need to explicitly clear BSS at boot, as that is required by the ELF ABI contract