ARC: Adjustments for gcc 4.8

* Force DWARF2 compliant unwind info in .debug_frame (vs. default DW4)
  as kernel unwinder can only grok DW2.
* Side-effect of above is additional debug_info which we discard now
* New gcc already uses arc600 multilibs for -mno-mpy
* switch to using uclibc compiler (to get -mmedium-calls and -mno-sdata)
  and also since buildroot can only use 1 toolchain

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>