Fix delivery of unaligned access exceptions.

In the unaligned access exception vector, actually pass the return PC
in the exception frame.  This is required in order for unaligned access
fixup handlers in the operating system to work.

Signed-off-by: Jason Thorpe <thorpej@me.com>
Message-Id: <20210603035317.6814-3-thorpej@me.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>