Since $(AS) is the assembler, passing it a list of preprocessor include
flags doesn't make much sense. The files aren't preprocessed which means
`#include` lines aren't respected, and while it would affect `.include`
usage, we never use that, and it's extremely unlikely to change. Plus,
it's extremely unlikely we'd have .s files in common places to include vs
contained entirely within a specific arch dir, and at that point, it can
be included directly (with no flags), or the arch can add the unique set
of include paths that it needs for itself.
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