This provides the bootstrap and low level helper functions for an
aarch64 kernel. We use semihosting to handle test output and exiting
the emulation. semihosting's parameter passing is a little funky so we
end up using the stack and pointing to that as the parameter block.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>