Fix PowerPC read_time()
Fix PowerPC read_time()

The inline assembly used for the PowerPC version of read_time() is
broken:

$ ./src/csmith |grep Seed
 * Seed:      276270
$ ./src/csmith |grep Seed
 * Seed:      276270
$ ./src/csmith |grep Seed
 * Seed:      276270

Since platform_gen_seed() only returns a long we can simplify things
and just use mftb. With the patch applied we see correct behaviour:

$ ./src/csmith |grep Seed
 * Seed:      1186590549319330
$ ./src/csmith |grep Seed
 * Seed:      1186590989534520
$ ./src/csmith |grep Seed
 * Seed:      1186591288978819

natgla/csmith,Panos90/csmith,Panos90/csmith,natgla/csmith,Panos90/csmith,natgla/csmith,natgla/csmith,Panos90/csmith