Remove reference to (unsupported, never used by Apple) UNWIND_ARM64_MODE_FRAME_OLD.

This constant was removed in recent 10.10 / iOS 8 SDKs; the frame mode
was never supported by PLCrashReporter, and as far as I'm aware, never
used in externally shipping iOS/ARM64 binaries.

Issue: PLCR-547
Fix ifdef conditional used to enable UNWIND_ARM64_MODE_* compatibility defines.

In the 10.10 and iOS 8 SDKs, CPU_TYPE_ARM64 is now defined, but
UNWIND_ARM64_MODE_* is not; rather than relying on CPU_TYPE_ARM64, we
should just directly test UNWIND_ARM64_MODE_MASK.

Issue: PLCR-547
Fix usage of the no-longer-constant PAGE_SIZE value in our unit tests.

In the iOS 8 SDK, this has been redefined to vm_page_size;

Issue: PLCR-547
Add missing mach/mach.h include required for PAGE_SIZE with the ARM iOS 8 SDK.

Issue: PLCR-547
Add missing include for mprotect() when building against the iOS SDK.

Issue: PLCR-547
Fix UNWIND_ARM64_MODE_* compatibility behavior for iOS targets.

These constants are defined as enums on iOS, preventing a stable #ifdef
check. As such, we always define these values on any Mac OS X target,
potentially overriding the existing enum identifiers.

To ensure this doesn't break inclusion of the header in which the enum
identifiers may be defined, we explicitly include the compact
unwind header.

Issue: PLCR-547