clang: make aarch64 target work again

In the ports clang, we had support for the AArch64 target.  When
we imported it into base, we tried to keep that status.

Even though we compile the libraries and have a patch in clang,
which enables an aarch64 Bitrig target, we do not enable that
in compile-time.  cmake used to take care of that in pots.

Therefore, enable the AArch64 target in the config files and
make the llvm binaries depend on those libraries.

No objections from pedro@.
arm: architected timer uses wrong tick value
arm: dynamically read architected timer frequency

On platforms that support it, use the timer frequency stored
in the cp15 timer register.  On devices that do not support
it (e. g. an exynos), use the agtimer_freq.

In the future we want to use the FDT instead of that variable.
Fix high capacity (> 2GB) eMMC support.
Based on a diff by Cedric Tessier, nezetic at gmail dot com, thanks!

Discussed with and ok jsg@

Imported to Bitrig by patrick@, no objections from natano@.
Add keys for Bitrig 1.0.
Add keys for Bitrig 1.1.
Crank to 1.0-beta.
Let tradcpp define __Bitrig__.

pointed out by and ok drahn@