tile: check error properly for vDSO calls

The tile vDSO vsyscalls were not properly setting the error value.
Conventionally, tile returns the same "non-negative success, negative
errno" value that x86 does (in r0), but it also returns "zero or positive
errno" in r1, which is what the regular syscall code checks.  This change
uses that convention for the vDSO calls as well.