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

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

Bugfix: feenableexcept() and fedisableexcept() should just return the
old exception mask, not mask | ~FE_ALL_EXCEPT.

MFC after: 2 weeks

Use #include "fenv.h" instead of #include <fenv.h>. This makes it
more convenient to compile the math library by itself.

Requested by: bde

Provide external definitions of all of the standardized functions in
fenv.h that are currently inlined.

The definitions are provided in fenv.c via 'extern inline'
declaractions. This assumes the compiler handles 'extern inline' as
specified in C99, which has been true under FreeBSD since 8.0.

The goal is to eventually remove the 'static' keyword from the inline
definitions in fenv.h, so that non-inlined references all wind up
pointing to the same external definition like they're supposed to.
I am deferring the second step to provide a window where
newly-compiled apps will still link against old math libraries.
(This isn't supported, but there's no need to cause undue breakage.)

Reviewed by: stefanf, bde

Placate new binutils, by using 16-bit %ax instead of 32-bit %eax as an
argument for fnstsw. Explicitely specify sizes for the XMM control and
status word and X87 control and status words.

Reviewed by: das
Tested by: avg
MFC after: 2 weeks

Remove an unneeded fnstcw instruction.

Noticed by: bde

Replace fegetmask() and fesetmask() with feenableexcept(),
fedisableexcept(), and fegetexcept(). These two sets of routines
provide the same functionality. I implemented the former as an
undocumented internal interface to make the regression test easier to
write. However, fe(enable|disable|get)except() is already part of
glibc, and I would like to avoid gratuitous differences. The only
major flaw in the glibc API is that there's no good way to report
errors on processors that don't support all the unmasked exceptions.

Insert a missing '~' in feholdexcept(), so that it correctly clears
the exception flags in the mxcsr as well as the x87 FPU.

Add an fenv.h implementation for the amd64 port.

Reviewed by: standards@