[libc][math] Added tanhf function.

Correct rounding function. Performance ~2x faster than glibc analog.

Performance (llvm 12 intel):
```
CORE_MATH_PERF_MODE=rdtsc PERF_ARGS='' ./perf.sh tanhf
GNU libc version: 2.31
GNU libc release: stable
13.279
37.492
18.145
CORE_MATH_PERF_MODE=rdtsc PERF_ARGS='--latency' ./perf.sh tanhf
GNU libc version: 2.31
GNU libc release: stable
40.658
109.582
66.568
```

Differential Revision: https://reviews.llvm.org/D130780