Fix sysdeps/mips/dl-machine-reject-phdr.h build with GCC 16

Fix sysdeps/mips/dl-machine-reject-phdr.h build with GCC 16

Building for MIPS has been broken with GCC mainline since mid-July,
probably GCC commit 0eac9cfee8cb0b21de866a04d5d59685ab35208f "c, c++:
Extend -Wunused-but-set-* warnings [PR44677]", because the variable
perfect_match in elf_machine_reject_phdr_p is set unconditionally, but
only used if _MIPS_SIM == _ABIO32.  Mark it with __attribute__ ((unused)),
which seems cleaner in such a conditionally-used case than making all
other logic relating to this variable conditional.

Tested with build-many-glibcs.py (compilers build, which previously
failed) for mips64-linux-gnu.