For user_settings.h builds, .S assembly files need to include user_settings.h
in order to get the defines used by the build. However, a user_settings.h may
contain code only understood by a C compiler and not the assembler (e.g. a
typedef). This commit makes it so our autotools and CMake builds produce a file
user_settings_asm.h when doing a user_settings.h build. This generated header
contains only the preprocessor directives from the user_settings.h. As a result,
it can be safely included by our assembly code files.
