[llvm] [CodeGen][Mips] Explicit ELF output file format for MIPS tests (PR #114678)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The thumb of rule is: `-mtriple=mipsel` defaults to ELF, even if we don't write '-elf' or '-linux'. Yes, ELF is the favored object file format, which gets this shortcut, while other object file formats need a lengthy target triple.

https://github.com/llvm/llvm-project/pull/114678