[OpenBIOS] make unoptimized sparc64 build usable with linux kernel

[Igor Kovalenko]

Hi!

After doing some homework with sparc64-elf-gcc -O0 :) I made this
local change in sparc64-call-client.diff
It allocates larger stack frame so unoptimized code could save
arguments on stack, and then calls of_client_interface().
This at least allows me to use debugger with qemu and unoptimized
sparc64 openbios build.