xfail two benchmarks that are failing on the bots

Author: brson

src/test/bench/shootout-fasta-redux.rs

@@ -1,3 +1,5 @@
+// xfail-test FIXME #5985 Doesn't typecheck on x86
+
 use core::cast::transmute;
 use core::from_str::FromStr;
 use core::libc::{FILE, STDOUT_FILENO, c_int, fdopen, fputc, fputs, fwrite};

src/test/bench/shootout-threadring.rs

@@ -10,6 +10,8 @@
 
 // Based on threadring.erlang by Jira Isa
 
+// xfail-test FIXME #5985 OOM's on the mac bot
+
 fn start(n_tasks: int, token: int) {
     let mut (p, ch1) = comm::stream();
     ch1.send(token);