[PATCH] powerpc: Don't blow away load_addr in start_thread

The patch to make process.c work for 32-bit and 64-bit
(06d67d54) broke some 64-bit binaries.
We were blowing away load_addr in gpr[2], so we weren't properly relocating
the entry point.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>