Use 4-byte slock_t on both PPC and PPC64.

Use 4-byte slock_t on both PPC and PPC64.

Previously we defined slock_t as 8 bytes on PPC64, but the TAS assembly
code uses word-wide operations regardless, so that the second word was
just wasted space.  There doesn't appear to be any performance benefit
in adding the second word, so get rid of it to simplify the code.