arc: Eliminate unused code and data with GCC's garbage collector

Finally GCC's garbage collector works on ARC so let's use it.
That's what I may see for HSDK:

Before:
   text	   data	    bss	    dec	    hex	filename
 290153	  10068	 222616	 522837	  7fa55	u-boot

After:
   text	   data	    bss	    dec	    hex	filename
 261999	   9460	 222360	 493819	  788fb	u-boot

Overall ~5% of memory footprint saved.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>