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

Add more tests for log functions. A few are commented out because the
long double versions don't pass yet. (They are rather nit-picky cases,
so there's ongoing discussion with Bruce about whether it is worth the
performance cost.)

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.

Add some tests for corner cases of log() in unusual rounding modes.
I wrote these ages ago, but they've been failing until now.

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

Add regression tests for logarithmic functions in the math library.

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.