build(xbuild): explicit support for compile output type

diff --git a/core/embed/upymod/build.rs b/core/embed/upymod/build.rs
index 48071270..7a90747a 100644
--- a/core/embed/upymod/build.rs
+++ b/core/embed/upymod/build.rs
@@ -8,8 +8,8 @@ use std::{
 };
 
 use xbuild::{
-    CLibrary, InputFiles, Result, WrapErr, bail, bail_unsupported, current_model_id, ensure,
-    model_ids,
+    CLibrary, InputFiles, OutputType, Result, WrapErr, bail, bail_unsupported, current_model_id,
+    ensure, model_ids,
 };
 
 fn main() -> Result<()> {
@@ -453,8 +453,8 @@ impl<'a> MpyBuilder<'a> {
         // the preprocessed output in corresponding .upydef files next to
         // each object file.
         let upydefs = self.lib.process_sources(
-            "upydef",
-            Some(&["-E", "-DNO_QSTR", "-DN_X64", "-DN_X86", "-DN_THUMB"]),
+            OutputType::Preprocessed("upydef"),
+            Some(&["-DNO_QSTR", "-DN_X64", "-DN_X86", "-DN_THUMB"]),
             Some(&extra_sources),
         )?;
 
diff --git a/core/embed/xbuild/src/clibrary/compile.rs b/core/embed/xbuild/src/clibrary/compile.rs
index b84c1fe3..6276942e 100644
--- a/core/embed/xbuild/src/clibrary/compile.rs
+++ b/core/embed/xbuild/src/clibrary/compile.rs
@@ -29,6 +29,21 @@ struct CompileUnit {
     attrs: Option<CompileAttrs>,
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum OutputType {
+    Object,
+    Preprocessed(&'static str),
+}
+
+impl OutputType {
+    pub fn extension(&self) -> &'static str {
+        match self {
+            OutputType::Object => "o",
+            OutputType::Preprocessed(ext) => ext,
+        }
+    }
+}
+
 // Represents the result of compiling a single source file
 struct CompileArtifact {
     // Index of the compile unit, used to maintain the original order of units
@@ -52,7 +67,7 @@ impl CompileUnit {
             cmd.arg("-MMD").arg("-MF").arg(cc_dep);
         }
 
-        cmd.arg("-c").arg("-o").arg(&self.output).arg(&self.input);
+        cmd.arg("-o").arg(&self.output).arg(&self.input);
 
         run_command_with_cc_dep(
             &mut cmd,
@@ -83,7 +98,7 @@ impl CLibrary {
 
         measure_time(format!("@@ {} compiled in", lib_name), || {
             // Run parallel build on all sources
-            let objects = self.process_sources("o", None, None)?;
+            let objects = self.process_sources(OutputType::Object, None, None)?;
             // Append manually added objects (e.g., vendor header)
             let objects = objects
                 .into_iter()
@@ -110,7 +125,7 @@ impl CLibrary {
     /// A vector of paths to the generated output files.
     pub fn process_sources(
         &self,
-        output_ext: &str,
+        output_type: OutputType,
         extra_args: Option<&[&str]>,
         extra_sources: Option<&[PathBuf]>,
     ) -> Result<Vec<PathBuf>> {
@@ -132,10 +147,10 @@ impl CLibrary {
         for (index, (src, attrs)) in sources.into_iter().enumerate() {
             // Derive absolute paths for input and output files
             let input = join_paths_lexically(&base_dir, &src);
-            let output = derive_output_path(&base_dir, &src, &out_dir, output_ext);
+            let output = derive_output_path(&base_dir, &src, &out_dir, output_type.extension());
 
             // Only generate .d files for object files compiled from C/C++ sources
-            let cc_dep = if output_ext == "o"
+            let cc_dep = if matches!(output_type, OutputType::Object)
                 && src
                     .extension()
                     .is_some_and(|ext| ext == "c" || ext == "cpp" || ext == "cc")
@@ -166,6 +181,15 @@ impl CLibrary {
             }
         }
 
+        match output_type {
+            OutputType::Object => {
+                attrs.add_flag("-c");
+            }
+            OutputType::Preprocessed(..) => {
+                attrs.add_flag("-E");
+            }
+        }
+
         // Compile all units in parallel
         let artifacts = compile_parallel(units, &attrs)?;
         let outputs = artifacts.into_iter().map(|a| a.output).collect();
diff --git a/core/embed/xbuild/src/lib.rs b/core/embed/xbuild/src/lib.rs
index 9e3f67bb..63acd64a 100644
--- a/core/embed/xbuild/src/lib.rs
+++ b/core/embed/xbuild/src/lib.rs
@@ -8,6 +8,7 @@ mod trezor;
 
 pub use attrs::CompileAttrs;
 pub use clibrary::CLibrary;
+pub use clibrary::compile::OutputType;
 pub use input_files::InputFiles;
 
 pub use dep_tracking::format_command_error;