mips: fix errno setting after syscall

mips: fix errno setting after syscall

If there was an error during syscall then after it's completion a3
register holds a non-zero value and v0 holds an actual error code which
should be saved in errno. This can be achieved by calling
__syscall_error with the value from v0 as a parameter. So this value
should be stored in a0, but the appropriate assembly instructions are
missing. Fixed this now by adding "move a0, v0".

I think it was once fixed by 58c5f8ba4cdf62342d05a546d15404cbbb3c4e07
but then something went wrong. Again...

Signed-off-by: Alexander Gordeev <lasaine@lvk.cs.msu.su>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>