Workaround for sparc64: bootstrap segfaults without -O3

Index: src/bootstrap/bootstrap.py
--- src/bootstrap/bootstrap.py.orig
+++ src/bootstrap/bootstrap.py
@@ -1114,6 +1114,9 @@ class RustBuild(object):
         else:
             env["RUSTFLAGS"] = "-Zallow-features="
 
+        if self.build_triple() == "sparc64-unknown-openbsd":
+            env["RUSTFLAGS"] += " -Copt-level=3"
+
         if not os.path.isfile(self.cargo()):
             raise Exception("no cargo executable found at `{}`".format(self.cargo()))
         args = [
