target-mips: Fix accumulator selection for MIPS16 and microMIPS

Add accumulator arguments to gen_HILO and gen_muldiv, rather than
extracting the accumulator directly from ctx->opcode.  The extraction
was only right for the standard encoding: MIPS16 doesn't have access
to the DSP registers, while microMIPS encodes the accumulator register
in a different field (bits 14 and 15).

Passing the accumulator register is probably an over-generalisation
for division and 64-bit multiplication, which never access anything
other than HI and LO, and which always pass 0 as the new argument.
Separating them felt a bit fussy though.

Signed-off-by: Richard Sandiford <rdsandiford@googlemail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>