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

Factor out some common code from the libm tests. This is a bit messy
because different tests have different ideas about what it means to be
"close enough" to the right answer, depending on the properties of the
function being tested. In the process, I fixed some warnings and
added a few more 'volatile' hacks, which are sufficient to make all
the tests pass at -O2 with clang.

signbit() returns nonzero for negative arguments, but we shouldn't assume
that it always returns the same nonzero value.

There are 3 tests here, not 2.

expm1(-big) generates an inexact exception but not underflow.

Add some regression tests for libm's exponential functions. These
mostly just test corner cases rather than accuracy. Some of the
tests don't pass right now if you compile libm at -O2 due to gcc
constant-folding some things that it shouldn't. I'll fix that
shortly.