include: headers: Replace __ASSEMBLY__ with __ASSEMBLER__

GCC has already a predefined macro __ASSEMBLER__ therefore, it can be
used without the need to define a new flag with -D__ASSEMBLY__.
This is useful when adding the library to projects having a build
system such one can build without the need to make changes.
THe build system does not use the Makefile in the sources tree.

Signed-off-by: Marouene Boubakri <marouene.boubakri@nxp.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>