[llvm-objcopy] Fix handling of MIPS64 little endian files

MIPS64 little endian target has a "special" encoding of `r_info`
relocation record field. Instead of one 64-bit little endian number, it
is a little endian 32-bit number followed by a 32-bit big endian number.
For correct reading and writing such fields we must provide information
about target machine into the corresponding routine. This patch does
this for the `llvm-objcopy` tool and fix handling of MIPS64 little
endian files.

The bug was reported in the issue #52647.

Differential Revision: https://reviews.llvm.org/D115635