The syscall return path on ppc64 checks if the error is between
-LAST_ERRNO and 0, if it is, does the usual inversion along with
setting a CR bit indicating to glibc that an error occured.

However, we had an interesting bug where we used a 32 bits logica
(unsigned) comparison, thus possibly doing false positives for
valid 64 bits unsigned values whose low 32 bits happen to be in
the error range.

Fix that.