New compiler warnings

[marxin]

Just small issues:

[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:467:1:   required from here
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:63:10: warning: variable 'check' set but not used [-Wunused-but-set-variable]
[  228s]    63 |     auto check = [&](i64 val, i64 lo, i64 hi) {
[  228s]       |          ^~~~~
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:108:9: warning: unused variable 'P' [-Wunused-variable]
[  228s]   108 |     u64 P = get_addr() + rel.r_offset;
[  228s]       |         ^
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:47:14: warning: variable 'dynrel' set but not used [-Wunused-but-set-variable]
[  228s]    47 |   ElfRel<E> *dynrel = nullptr;
[  228s]       |              ^~~~~~
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc: In instantiation of 'void mold::elf::InputSection<E>::apply_reloc_alloc(mold::elf::Context<E>&, mold::u8*) [with E = mold::elf::MIPS64BE; mold::u8 = unsigned char]':
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:468:1:   required from here
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:63:10: warning: variable 'check' set but not used [-Wunused-but-set-variable]
[  228s]    63 |     auto check = [&](i64 val, i64 lo, i64 hi) {
[  228s]       |          ^~~~~
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:108:9: warning: unused variable 'P' [-Wunused-variable]
[  228s]   108 |     u64 P = get_addr() + rel.r_offset;
[  228s]       |         ^
[  228s] /home/abuild/rpmbuild/BUILD/mold-2.0.0/elf/arch-mips64.cc:47:14: warning: variable 'dynrel' set but not used [-Wunused-but-set-variable]
[  228s]    47 |   ElfRel<E> *dynrel = nullptr;
[  228s]       |              ^~~~~~