Update project for Xcode 5, with iOS/arm64 enabled.

Issue: PLCR-495
Update comment; there's now a 64-bit ARM iOS device.

iOS still doesn't support the mach_* APIs, but as noted in the new
comment, this remains unneccesary insofar as out-of-process targeting of
processes remains unsupported by Apple.

Issue: PLCR-495
Merge branch 'master' into landonf-PLCR-495-armv8
Remove -Wno-tautological-constant-out-of-range-compare; this is now handled with #pragma clang diagnostic.
Merge branch 'master' into landonf-PLCR-495-armv8
Include arm64 in the global list of supported architectures.
Fix incorrect implicit conversion of 64-bit size value.
Define ARM64's natural alignment.

Issue: PLCR-495
Add -Wno-shorten-64-to-32 to allow the project to build for arm64 with clang-500.2.75

there remains code in the project (third-party protobuf code,
in/out-of-process 32-bit/64-bit stack walking) that assumes implicit
64->32 casts are permitted, with the expectation of overflow.

We'll need to re-enable this warning later and revisit the specific
cases in question, either adding an explicit cast, or handling any cases
where overflow is theoretically possible and undesired.
Stub out the ARM64 thread state APIs.
Begin migrating to the new unified ARM thread state API.

This also includes a work-around for rdar://14970271, as documented
in the inline comments.
Remove now unused arm64 thread implementations.
Implement the ARM64 high-level thread state API.

Issue: PLCR-495
Add an ARM64 plcrash_async_thread_state_current() assembly trampoline.

Since we don't have ARM64 hardware, this remains untested, and remains
to be validated against real hardware.

Issue: PLCR-495
Fix 64-bit compatibility issues that break the build with clang-500.2.75

Issue: PLCR-495
Enable the ARM thread state code for arm64

Issue: PLCR-495
Add an unwind test harness for arm64.

Issue: PLCR-495
Enable ARM64 support in the frame walker.

Issue: PLCR-495
Fix remaining arm64 unit test build issues.

Issue: PLCR-495
Remove duplicate tests.

These unit tests (and the APIs they test) were moved to the
PLCrashAsyncThread module. The original tests were never deleted
after the code was moved.

The replacement tests may be found with the same names in PLCrashAsyncThreadTests.