Round sqrt correctly in the current rounding mode

[ibmibmibm]

• IEEE 754-2019 4.3 and 5.4.1: sqrt gives the infinitely precise root
  rounded in the current mode. The 32 and 64 bit kernels truncated the
  integer root, an odd exponent multiplied the root by a rounded sqrt(10)
  for the three sizes, and no kernel read the mode.
• For an odd exponent the three kernels take the root of 10 * gx, so the
  result is rounded once. The estimate is scaled by an integer sqrt(10),
  and the Newton steps correct it. The multiply by sqrt10_v goes away.
• A new helper reads the mode, and the final step of each kernel keeps
  the floor, steps up on a remainder, or steps up past the half point.
• The step down to the floor is a loop. One Newton step can land two
  above the floor when the estimate is off and the root ends near .999.
• An odd power of ten takes the kernel path too. The old fast path gave
  sqrt(0.1) one step low, and test_sqrt.cpp expected that value.
• A new test compares sqrt against the floor and the ceiling of the root
  for the six types in the five modes, with a sibling under the
  compile-time upward mode.

Fixes #1462

[mborland]

LGTM. Thanks as always