Most files in arch/um have "Licensed under the GPL" in comments at the top. Convert to use SPDX headers. I've broken these changes into a patch series even though they're effectively one logical change, because the diff was >1700 lines. I can resend as a single patch if preferred.
There are some remaining files where there was no comment indicating what the license was, so I haven't added SPDX tags for those (though presumably they're also GPLv2 -- I can add tags for them in a separate patch if you like). The remaining files without SPDX tags are: arch/um/kernel/asm-offsets.c arch/um/kernel/dyn.lds.S arch/um/kernel/vmlinux.lds.S arch/um/include/shared/mem_user.h arch/um/include/asm/vmlinux.lds.h arch/um/include/asm/unwind.h arch/um/include/asm/kvm_para.h arch/um/include/asm/asm-prototypes.h arch/um/os-Linux/execvp.c arch/um/configs/x86_64_defconfig arch/um/configs/i386_defconfig arch/um/drivers/random.c arch/um/drivers/harddog_kern.c
