[libc][math][c23] Improve rsqrtf16 function for targets without fp32 FPUs. (#160639)

Closes #159378

#### Changes
- This PR adds math approximation for targets that don't have hardware
for floats - in other words, targets that don't have
`LIBC_TARGET_CPU_HAS_FPU_FLOAT`
- This PR also introduces Google Benchmark for rsqrtf16
- Fixed typo in `+inf` case. Should return +0 according to
[F.10.4.9](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf)

GitOrigin-RevId: e4a0e1dfcb5e23e1e5acf7e3521cea840e4383d6