Fix build of sincosl on machines where long double == double

ok deraadt@

Implement sicos(3), sincosf(3) and sincosl(3). These functions are common
extensions and modern compilers (such as clang) will use them to optimize
separate calculations of sine and cosine.

ok tom@

Implement sicos(3), sincosf(3) and sincosl(3). These functions are common
extensions and modern compilers (such as clang) will use them to optimize
separate calculations of sine and cosine.

ok tom@, patrick@, deraadt@, jmc@

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