unwind: make build.rs do clean rebuilds

If the build by this was interrupted or otherwise left incomplete, there
could be object files hanging around, which triggers an assertion later.
Since we unconditionally rebuild everything here anyway, start with a
clean slate each build.

library/unwind/build.rs

@@ -65,8 +65,8 @@ fn main() {
 }
 
 mod llvm_libunwind {
-    use std::env;
-    use std::path::Path;
+    use std::path::{Path, PathBuf};
+    use std::{env, fs, io};
 
     /// Compile the libunwind C/C++ source code.
     pub fn compile() {
@@ -76,6 +76,14 @@ mod llvm_libunwind {
         let mut cpp_cfg = cc::Build::new();
         let root = Path::new("../../src/llvm-project/libunwind");
 
+        // We depend on starting with a fresh build directory each time.
+        let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+        if let Err(x) = fs::remove_dir_all(&out_dir) {
+            if x.kind() != io::ErrorKind::NotFound {
+                panic!("Failed removing OUT_DIR at {}: {}", out_dir.display(), x);
+            }
+        }
+
         cpp_cfg.cpp(true);
         cpp_cfg.cpp_set_stdlib(None);
         cpp_cfg.flag("-nostdinc++");
@@ -137,7 +145,7 @@ mod llvm_libunwind {
             "UnwindRegistersSave.S",
         ];
 
-        let cpp_sources = vec!["Unwind-EHABI.cpp", "Unwind-seh.cpp", "libunwind.cpp"];
+        let cpp_sources = &["Unwind-EHABI.cpp", "Unwind-seh.cpp", "libunwind.cpp"];
         let cpp_len = cpp_sources.len();
 
         if target.contains("x86_64-fortanix-unknown-sgx") {