R28 is used as the SB register on MIPS64, and it was printed as
"RSB" on both 32-bit and 64-bit MIPS. This is confusing on MIPS32
as there R28 is just a general purpose register. Further, this
string representation is used in the assembler's frontend to parse
register symbols, and this leads to failure in parsing R28 in
MIPS32 assembly code. Change rconv to always print the register
as R28. This fixes the parsing problem on MIPS32, and this is
a reasonable representation on both MIPS32 and MIPS64.

Change-Id: I30d6c0a442fbb08ea615f32f1763b5baadcee1da
Reviewed-on: https://go-review.googlesource.com/92915
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>