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

Replace two lines accidentally removed in r226218. Thanks to bde
for noticing this.

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

Make the fenv.h routines work for programs that use SSE for
floating-point arithmetic on i386. Now I'm going to make excuses
for why this code is kinda scary:

- To avoid breaking the ABI with 5.3-RELEASE, we can't change
sizeof(fenv_t). I stuck the saved mxcsr in some discontiguous
reserved bits in the existing structure.

- Attempting to access the mxcsr on older processors results
in an illegal instruction exception, so support for SSE must
be detected at runtime. (The extra baggage is optimized away
if either the application or libm is compiled with -msse{,2}.)

I didn't run tests to ensure that this doesn't SIGILL on older 486's
lacking the cpuid instruction or on other processors lacking SSE.
Results from running the fenv regression test on these processors
would be appreciated. (You'll need to compile the test with
-DNO_STRICT_DFL_ENV.) If you have an 80386, or if your processor
supports SSE but the kernel didn't enable it, then you're probably out
of luck.

Also, I un-inlined some of the functions that grew larger as a result
of this change, moving them from fenv.h to fenv.c.

Add an fenv.h implementation for the i386 port.

Reviewed by: standards@