target/arm: Plumb FEAT_RPRES frecpe and frsqrte through to new helper

target/arm: Plumb FEAT_RPRES frecpe and frsqrte through to new helper

FEAT_RPRES implements an "increased precision" variant of the single
precision FRECPE and FRSQRTE instructions from an 8 bit to a 12
bit mantissa. This applies only when FPCR.AH == 1. Note that the
halfprec and double versions of these insns retain the 8 bit
precision regardless.

In this commit we add all the plumbing to make these instructions
call a new helper function when the increased-precision is in
effect. In the following commit we will provide the actual change
in behaviour in the helpers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>