arc: clean-up init procedure

Intention behind this work was elimination of as much assembly-written
code as it is possible.

In case of ARC we already have relocation fix-up implemented in C so why
don't we use C for U-Boot copying, .bss zeroing etc.

It turned out x86 uses pretty similar approach so we re-used parts of
code in "board_f.c" initially implemented for x86.

Now assembly usage during init is limited to stack- and frame-pointer
setup before and after relocation.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Simon Glass <sjg@chromium.org>