I've committed this patch, intended for bare-metal nios2-elf targets 
with custom linker scripts.  In conjunction with section attributes, it 
provides a way to tell GCC that an object is assigned to the low or high 
32K memory regions so that it can be addressed via a signed 16-bit 
offset from r0, the zero register.  From GCC's perspective it's quite 
similar to GP-relative addressing except using a different base 
register.  The output code uses the existing %lo relocation so no 
assembler or linker changes are necessary to support this.

-Sandra