math: Only build internal functions on non-ARM platforms
softmath: Add cosl and error handling for cos functions
softmath: Add sinl and error handling for sin functions
softmath: Add log and logl
softmath: Add log10l
Unusable header fix-ups

IOhannes m zmoelnig reported (via https://bugs.debian.org/761191) that a few
headers are missing the leading / in their boilerplate, which renders them
unusable. The attached patch (against master) fixes this; it applies to
stable too.

Patch by Stephen Kitt -- skitt at debian.org