I never heard of .sx but google lead to this:

As an alternative to using .S, the suffix .sx is recognized for this purpose (starting with GCC 4.3.0). This is primarily meant to be compatible with other compiler environments that have been providing this variant before in order to cope with operating systems where filenames are case-insensitive (and, with some versions of make that could not distinguish between .s and .S on such systems).

If it doesn't clash, I see no reason not to also add it.

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