Re-enable clock_gettime and gettimeofday

Unfortunately, now firefox is failing to start up under recording

    rr: /home/cjones/rr/rr/src/share/util.c:1112: inject_and_execute_syscall: Assertion `((0xFF0000 & ctx->status) >> 16) == 0' failed.

but the tests are passing, so I declare progress! ;)