Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.Approved by: re (implicit)Sponsored by: The FreeBSD Foundation
Move the macros GET_LDBL_EXPSIGN() and SET_LDBL_EXPSIGN() intomath_private.h, so they can be used elsewhere in the math library.
Remove a (bogus) remnant of debugging this on sparc64.
Implement hypotl(). This is bde's conversion of fdlibm hypot(), with minorfixes for ld128 by me.