target-mips: Fix ALIGN instruction when bp=0

target-mips: Fix ALIGN instruction when bp=0

If executing ALIGN with shift count bp=0 within mips64 emulation,
the result of the operation should be sign extended.

Taken from the official documentation (pseudo code) :

ALIGN:
	tmp_rt_hi = unsigned_word(GPR[rt]) << (8*bp)
	tmp_rs_lo = unsigned_word(GPR[rs]) >> (8*(4-bp))
	tmp = tmp_rt_hi || tmp_rt_lo
	GPR[rd] = sign_extend.32(tmp)

Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>