Compile genprime-swift with -O3

Didn't realize the C-family implementations were being compiled with -O3. I added this for Swift as well and the results changed quite dramatically.

Benchmarks:

```
ganglion:genprime eddie$ ./genprime-swift 50000 500000
Found    50000 primes in    0.159726 seconds (last was     611953)
Found   100000 primes in    0.450983 seconds (last was    1299709)
Found   150000 primes in    0.835602 seconds (last was    2015177)
Found   200000 primes in    1.291085 seconds (last was    2750159)
Found   250000 primes in    1.821854 seconds (last was    3497861)
Found   300000 primes in    2.392536 seconds (last was    4256233)
Found   350000 primes in    3.027330 seconds (last was    5023307)
Found   400000 primes in    3.712356 seconds (last was    5800079)
Found   450000 primes in    4.445630 seconds (last was    6581963)
Found   500000 primes in    5.264614 seconds (last was    7368787)
```

```
ganglion:genprime eddie$ ./genprime-c 50000 500000
Found    50000 primes in    0.162000 seconds (last was     611953)
Found   100000 primes in    0.440510 seconds (last was    1299709)
Found   150000 primes in    0.803440 seconds (last was    2015177)
Found   200000 primes in    1.243230 seconds (last was    2750159)
Found   250000 primes in    1.744320 seconds (last was    3497861)
Found   300000 primes in    2.298760 seconds (last was    4256233)
Found   350000 primes in    2.904560 seconds (last was    5023307)
Found   400000 primes in    3.585480 seconds (last was    5800079)
Found   450000 primes in    4.271710 seconds (last was    6581963)
Found   500000 primes in    5.017150 seconds (last was    7368787)
```

Also, a binary size comparison between Swift and C (both compiled with -O3):

```
ganglion:genprime eddie$ ls -al genprime-{swift,c}
-rwxr-xr-x  1 eddie  staff    8720 Jun  4 16:31 genprime-c
-rwxr-xr-x  1 eddie  staff  162520 Jun  8 12:40 genprime-swift
```

For completeness, Swift's binary size compiled without -O3:

```
ganglion:genprime eddie$ ls -al genprime-swift
-rwxr-xr-x  1 eddie  staff  50736 Jun  4 16:32 genprime-swift
```

Signed-off-by: Eddie Ringle <eddie@eringle.net>