[LLD] [MachO] Fix GCC build warnings

This fixes the following warnings produced by GCC 9:

    ../tools/lld/MachO/Arch/ARM64.cpp: In member function ‘void {anonymous}::OptimizationHintContext::applyAdrpLdr(const lld::macho::OptimizationHint&)’:
    ../tools/lld/MachO/Arch/ARM64.cpp:448:18: warning: comparison of integer expressions of different signedness: ‘int64_t’ {aka ‘long int’} and ‘uint64_t’ {aka ‘long unsigned int’} [-Wsign-compare]
      448 |   if (ldr.offset != (rel1->referentVA & 0xfff))
          |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    ../tools/lld/MachO/UnwindInfoSection.cpp: In function ‘bool canFoldEncoding(compact_unwind_encoding_t)’:
    ../tools/lld/MachO/UnwindInfoSection.cpp:404:44: warning: comparison between ‘enum<unnamed>’ and ‘enum<unnamed>’ [-Wenum-compare]
      404 |   static_assert(UNWIND_X86_64_MODE_MASK == UNWIND_X86_MODE_MASK, "");
          |                                            ^~~~~~~~~~~~~~~~~~~~
    ../tools/lld/MachO/UnwindInfoSection.cpp:405:49: warning: comparison between ‘enum<unnamed>’ and ‘enum<unnamed>’ [-Wenum-compare]
      405 |   static_assert(UNWIND_X86_64_MODE_STACK_IND == UNWIND_X86_MODE_STACK_IND, "");
          |                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~

Differential Revision: https://reviews.llvm.org/D130970