microblaze: Enable GCC garbage collector for full U-Boot

GCC's garbage collector works for Microblaze for quite a long time but none
has enabled it.
The same change has be done for example by commit fac47904 ("arc:
Eliminate unused code and data with GCC's garbage collector").

Before:
   text	   data	    bss	    dec	    hex	filename
 588760	  33592	  39192	 661544	  a1828	u-boot

After:
   text	   data	    bss	    dec	    hex	filename
 504504	  32164	  38608	 575276	  8c72c	u-boot

Which saves almost 15% of memory footprint.

Also group symbols/functions to proper section.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>