sparc: fix openfirmware entry point.

According to the IEEE standard documentation, the OpenFirmware entry
point should be in %o3. But that doesn't work, and both FreeBSD and
NetBSD expect it in %o4

sparc: fix openfirmware entry point.

According to the IEEE standard documentation, the OpenFirmware entry
point should be in %o3. But that doesn't work, and both FreeBSD and
NetBSD expect it in %o4 (5th argument of the function). I suspect this
was changed for 64bit sparc, but neither the sparc nor 64bit
openfirmware specs mention it.

Move the sparc and powerpc specific parts out of the generic start.cpp
for openfirmware as they each have some specificities. More
specifically:
- sparc already clears bss for us
- entry point arguments are different
- determine_machine is of course platform specific

Change-Id: Icaa05087e88ea4d29198e3565223459aed75cdf9
Reviewed-on: https://review.haiku-os.org/c/1470
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

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