[arch][riscv] make sure zero sized bsses work
[arch][riscv] make sure zero sized bsses work

-Skip looping if the bss size is actually zero

Issue #288

littlekernel/lk,littlekernel/lk,hollanderic/lkstuff,hollanderic/lkstuff,littlekernel/lk,hollanderic/lkstuff,littlekernel/lk,hollanderic/lkstuff