MIPS: start.S: optimize BSS initialization

Get the start and end address for clearing BSS from the newly
introduced symbols __bss_start and __bss_end. After GOT is
relocated, those symbols are already pointing to the correct
addresses.

Also optimize the loop by moving the address incrementation
to the delay slot to avoid the initial sub instruction.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>