Refactor prng to not use 64-bit atomics on 32-bit platforms.

This resolves #495.
diff --git a/src/prof.c b/src/prof.c
index 4bafb39..19c8fb7 100644
--- a/src/prof.c
+++ b/src/prof.c
@@ -878,7 +878,7 @@
 	 *   pp 500
 	 *   (http://luc.devroye.org/rnbookindex.html)
 	 */
-	r = prng_lg_range(&tdata->prng_state, 53, false);
+	r = prng_lg_range_u64(&tdata->prng_state, 53);
 	u = (double)r * (1.0/9007199254740992.0L);
 	tdata->bytes_until_sample = (uint64_t)(log(u) /
 	    log(1.0 - (1.0 / (double)((uint64_t)1U << lg_prof_sample))))