[libm] Implementations for rsqrt[fl].

All,

I submitted https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=295089
which contains an implementation of rsqrt[fl](x)=1/sqrt[fl](x).  These
appear in C23 7.12.7.9 and F.10.4.9 as well as IEEE-754 2008.  Under
IEEE-754, the functions are required(?)/recommended to be correctly
rounded in all rounding modes.  I have only checked round-to-nearest.
Using my testing franework, I see

% ./tlibm rsqrt -fPED -x 0x1p-127 -X 0x1p126 -s 0
Interval tested for rsqrtf: [5.87747e-39,8.50706e+37]
1000000 calls, 0.011062 secs, 0.01106 usecs/call
        ulp <= 0.5: 100.000% 2118123393 | 100.000% 2118123393
0.5 <  ulp <  0.6:  0.000%     127 | 100.000% 2118123520
Max ulp: 0.500000 at 1.17549421e-38
%./tlibm rsqrt -fa  1.17549421e-38
    x =  1.17549421e-38f, /* 0x007fffff */
libm =  9.22337204e+18f, /* 0x5f000000 */
mpfr =  9.22337314e+18f, /* 0x5f000001 */
  ULP = 0.50000

%./tlibm rsqrt -dP -x 0 -X 0x1p1001 -N700 -s 0
Interval tested for rsqrt: [0,2.14302e+301]
700000000 calls, 9.325613 secs, 0.01332 usecs/call
        ulp <= 0.5: 100.000% 699999999 | 100.000% 699999999
Max ulp: 0.500000 at 1.2611313594916888e+301

% ./tlibm rsqrt -lP -x 0 -X 0x1p16380 -N700 -s 0
Interval tested for rsqrtl: [0,7.43582e+4930]
700000000 calls, 53.109852 secs, 0.07587 usecs/call
        ulp <= 0.5: 100.000% 699999999 | 100.000% 699999999
Max ulp: 0.500000 at 6.157706927493729414081e+4930

If a committer wants to pick up the diff and apply it
to src/  I can answer any questions you may have.
-- 
steve