compiler_types: Add __assume macro

compiler_types: Add __assume macro

Make the statement attribute "assume" with a new __assume macro available.

The assume attribute is used to indicate that a certain condition is
assumed to be true. Compilers may or may not use this indication to
generate optimized code. If this condition is violated at runtime, the
behavior is undefined.

Note that the clang documentation states that optimizers may react
differently to this attribute, and this may even have a negative
performance impact. Therefore this attribute should be used with care.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>