Hexagon (target/hexagon) add F2_sfinvsqrta

Hexagon (target/hexagon) add F2_sfinvsqrta

Rd32,Pe4 = sfinvsqrta(Rs32)
    Square root approx

The helper packs the 2 32-bit results into a 64-bit value,
and the fGEN_TCG override unpacks them into the proper results.

Test cases in tests/tcg/hexagon/multi_result.c
FP exception tests added to tests/tcg/hexagon/fpstuff.c

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <1617930474-31979-19-git-send-email-tsimpson@quicinc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>