[Object] Change ELFObjectFile<ELFT>::getFileFormatName() to use BFD names

Follow-up for D74433

What the function returns are almost standard BFD names, except that "ELF" is
in uppercase instead of lowercase.

This patch changes "ELF" to "elf" and changes ARM/AArch64 to use their BFD names.
MIPS and PPC64 have endianness differences as well, but this patch does not intend to address them.

Advantages:

* llvm-objdump: the "file format " line matches GNU objdump on ARM/AArch64 objects
* "file format " line can be extracted and fed into llvm-objcopy -O literally.
  (https://github.com/ClangBuiltLinux/linux/issues/779 has such a use case)

Affected tools: llvm-readobj, llvm-objdump, llvm-dwarfdump, MCJIT (internal implementation detail, not exposed)

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D76046