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

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

Make the order of operations for lib/msun more clear.

Tested with md5 sum of object code

Reported by: swildner@DragonFlyBSD.org
Submitted by: bde

Fix some regressions caused by the switch from gcc to clang. The fixes
are workarounds for various symptoms of the problem described in clang
bugs 3929, 8100, 8241, 10409, and 12958.

The regression tests did their job: they failed, someone brought it
up on the mailing lists, and then the issue got ignored for 6 months.
Oops. There may still be some regressions for functions we don't have
test coverage for yet.

Fix a regression introduced in r226371: When the high part of x*y
exactly cancels with z, return the low part of x*y instead of
discarding it.

Fix a double-rounding bug in fma{,f,l}. The bug would occur in
round-to-nearest mode when the result, rounded to twice machine
precision, was exactly halfway between two machine-precision
values. The essence of the fix is to simulate a "sticky bit" in
the pathological cases, which is how hardware implementations
break the ties.

MFC after: 1 month

Refactor this code by introducing separate functions to handle the
extra-precision add and multiply operations. This simplifies future
work but shouldn't result in any functional change.

Fix some corner cases:
- fma(x, y, z) returns z, not NaN, if z is infinite, x and y are finite,
x*y overflows, and x*y and z have opposite signs.
- fma(x, y, z) doesn't generate an overflow, underflow, or inexact exception
if z is NaN or infinite, as per IEEE 754R.
- If the rounding mode is set to FE_DOWNWARD, fma(1.0, 0.0, -0.0) is -0.0,
not +0.0.

Implement modfl().

Fix the double rounding problem with subnormals, and
remove the XXX comments, which no longer apply.

Implement fmal.