Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r226594:
Bugfix: feenableexcept() and fedisableexcept() should just return the
old exception mask, not mask | ~FE_ALL_EXCEPT.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

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@