[libc][Bazel] Add missing float MPFR math tests (#227081)
This PR adds missing Bazel targets for existing libc MPFR-based math
tests in:
`utils/bazel/llvm-project-overlay/libc/test/src/math/BUILD.bazel`
### Added targets
**Float math functions:**
- `acospif`
- `asinpif`
- `copysignf`
- `exp10m1f`
- `fmaf`
- `fmaxf`
- `fminf`
- `lgammaf`
- `logbf`
- `modff`
- `rsqrtf`
- `scalbnf`
**BFloat16 arithmetic functions:**
- `bf16addf`
- `bf16divf`
- `bf16fmaf`
- `bf16mulf`
- `bf16subf`
The targets use the appropriate existing MPFR test headers and
dependencies where required.
Co-authored-by: Zorojuro <sawantsukumar@gmail.com>
