Build: fix producesPdbFile logic (#15756)

Fixes bug causing ReleaseSmall to fail on Windows.

Due to the change in default behavior of ReleaseSmall, debug info are
stripped by default. However because `Compile.create` still defaults to
null, `producesPdbFile` will report true for
`lib/std/Build/Step/InstallArtifact.zig` causing it to fail on copying a
file that does not exist. This commit change the default of strip
depending on `optimize`.

lib/std/Build/Step/Compile.zig

@@ -691,7 +691,7 @@ pub fn isStaticLibrary(self: *Compile) bool {
 pub fn producesPdbFile(self: *Compile) bool {
     if (!self.target.isWindows() and !self.target.isUefi()) return false;
     if (self.target.getObjectFormat() == .c) return false;
-    if (self.strip == true) return false;
+    if (self.strip == true or (self.strip == null and self.optimize == .ReleaseSmall)) return false;
     return self.isDynamicLibrary() or self.kind == .exe or self.kind == .@"test";
 }