dlltool: file name too long

During the execution of the command: i686-w64-mingw32-dlltool
--input-def $def_filepath --output-delaylib $filepath --dllname qemu.exe
An error occurred:
i686-w64-mingw32-dlltool: failed to open temporary head file: ..._w64_mingw32_nativesdk_qemu_8_2_2_build_plugins_libqemu_plugin_api_a_h.s

Due to the path length exceeding the Linux system's file name length
limit (NAME_MAX=255), the temporary file name generated by the
i686-w64-mingw32-dlltool command becomes too long to open. To address
this, a new temporary file name prefix is generated using tmp_prefix =
prefix_encode ("d", getpid()), ensuring that the file name does not
exceed the system's length limit.

Signed-off-by: Jiaying Song <jiaying.song.cn@windriver.com>
Reviewed-by: Alan Modra <amodra@gmail.com>