Add fallback macros for missing Mach-O constants

[jriv01]

Fixes compilation failures on platforms that lack some Mach-O definitions:

src/macho.cc:79:30: error: use of undeclared identifier 'CPU_SUBTYPE_ARM64E'
  return subtype_proper == CPU_SUBTYPE_ARM64E;
                           ^
src/macho.cc:483:10: error: use of undeclared identifier 'LC_DYLD_CHAINED_FIXUPS'
    case LC_DYLD_CHAINED_FIXUPS:
         ^
src/macho.cc:486:10: error: use of undeclared identifier 'LC_DYLD_EXPORTS_TRIE'
    case LC_DYLD_EXPORTS_TRIE:
         ^
3 errors generated.

This change adds fallbacks at the top of macho.cc to ensure Bloaty can independently compile.