Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Eliminate __real__ and __imag__ gccisms.

Use cpack() and the gcc extension __imag__ to implement cimag() and
conj() instead of using expressions like z * I. The latter is bad for
several reasons:

1. It is implemented using arithmetic, which is unnecessary, and can
generate floating point exceptions, contrary to the requirements on
these functions.

2. gcc implements complex multiplication using a formula that breaks
down for infinities, e.g., it gives INFINITY * I == nan + inf I.

Add implementations for cimag{,f,l}, creal{,f,l} and conj{,f,l}. They are
needed for cases where GCC's builtin functions cannot be used and for
compilers that don't know about them.

Approved by: das (mentor)