[PEFIXUP] Recalculate checksums for kernel-mode images (reactos#8969)

lld can leave the PE checksum field zero for reproducible output. This is acceptable for user-mode images, but Windows rejects kernel-mode images without a valid checksum.

Keep preserving linker-produced zero checksums for other images, but force checksum recalculation when pefixup is operating on a kernel-mode image.

This was found while validating Clang-built amd64 kmtest drivers against Windows 7: the same driver source built with GCC loaded, while the Clang/lld-built .sys failed with ERROR_BAD_EXE_FORMAT / 0xC1. Recalculating the PE checksum made the Clang-built driver load and the tests run correctly.

Author: ahmedarif193

sdk/tools/pefixup.c

@@ -521,8 +521,17 @@ int main(int argc, char **argv)
 
     if (!result)
     {
-        /* Success. Recalculate the checksum only if this is not a reproducible build file */
-        if (nt_header->OptionalHeader.CheckSum != 0)
+        int is_kernel_image = (mode == MODE_KERNELDRIVER ||
+                               mode == MODE_WDMDRIVER ||
+                               mode == MODE_KERNELDLL ||
+                               mode == MODE_KERNEL);
+
+        /*
+         * Preserve linker-produced zero checksums unless this is a kernel-mode
+         * image, where the NT loader requires a valid checksum. This fixes
+         * lld-built drivers, which otherwise keep the reproducible zero value.
+         */
+        if (nt_header->OptionalHeader.CheckSum != 0 || is_kernel_image)
             fix_checksum(buffer, len, nt_header);
 
         /* We could optimize by only writing the changed parts, but keep it simple for now */