ARC: Adjustments for gcc 4.8

* Force generation of dearf2 in .debuf_frame (stack unwinding)
* -mno-mpy already uses arc600 multilib
* 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>