This directive is not implemented by clang assembler. The traditional
way to implement structs in assembly is using two macros for every field,
one for the offset, and another one for the size. For every field, the
offset can be calculated using the size and offset of the previous field.

Change-Id: Iacc6781e8f302fb925898737b8e85ab4e88a51cc
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>