Fix Darwin ARM64 same-page SIGBUS and private method patch corruption

• Fix a potential SIGBUS on Darwin ARM64 when write removes execute permission from its own active page (NOP isolation was only 4 KiB on 16 KiB pages)
• Fix private method patch corruption on Darwin ARM64: short methods could be overrun by a 24-byte jump; now use a 4-byte B plus an executable trampoline

Reference: #188