[llvm-objdump] Print file format in lowercase to match GNU output.

Summary:
GNU objdump prints the file format in lowercase, e.g. `elf64-x86-64`. llvm-objdump prints `ELF64-x86-64` right now, even though piping that into llvm-objcopy refuses that as a valid arch to use.

As an example of a problem this causes, see: https://github.com/ClangBuiltLinux/linux/issues/779

Reviewers: MaskRay, jhenderson, alexshap

Reviewed By: MaskRay

Subscribers: tpimh, sbc100, grimar, jvesely, nhaehnle, kerbowa, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74433