engine: Rename the magic slice assembly file

Assembly files are supposed to be run through the preprocessor only
when they end in a captial .S. On the linux build, it appears to run
through the preprocessor anyway, but this is not the case for the
windows build (note that our windows binary is cross-compiled from
linux; this is not about case-insensitivity of the windows
filesystem). The windows build is broken when this file has a
lowercase .s.

Renames that consist only of case changes are poorly supported by git
and other tools, so also change the base name of the file. Some tools
support a .sx extension for a less-subtle version of this distinction,
but go does not.

Release note: None