tests/tcg/alpha: add system boot.S

This provides the bootstrap and low level helper functions for an
alpha kernel.  We use direct access to the DP264 serial port for
test output, and hard machine halt to exit the emulation.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190501184306.15208-1-richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>