tests/tcg/ppc64: remove dependency on qemu/compiler.h

Replace definition of vector attribute, remove unlikely, and detect
endianness from __BYTE_ORDER__.

It's the only tests that use this, so for consistency, it's better to
not make exceptions. Also, not having this include path by default
prevents anyone from having the "good" idea to reuse any other header
from qemu in the future.

Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Aniket Sahu <asahu1x@linux.ibm.com>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Message-ID: <20260827221506.91574-76-pierrick.bouvier@oss.qualcomm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>