tests/tcg: fix semihosting SYS_EXIT for aarch64 in boot.S

tests/tcg: fix semihosting SYS_EXIT for aarch64 in boot.S

We don't expect to hit exceptions in our testing so currently all the
vectors report an un-expected exception and then attempt to exit.
However for aarch64 we should always use the extended information
block as we do in _exit. Rather than duplicate the code on the error
handler just branch to the _exit handler with a failing status code.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250404115641.258048-1-alex.bennee@linaro.org>