rustbuild: Pass -fPIC on 32-bit non-Windows platforms

This is a smaller and more targeted backport of rust-lang#39523 which drives to the heart
of the issue, just passing `-fPIC` on 32-bit platforms. More rationale for this
commit can be found in rust-lang#39523 itself.

Author: alexcrichton

src/bootstrap/lib.rs

@@ -812,6 +812,11 @@ impl Build {
             },
             _ => {},
         }
+
+        if !target.contains("windows") && target.contains("i686") {
+            base.push("-fPIC".into());
+        }
+
         return base
     }