[libc][stdlib] strfromf does not handle inf correctly

Calling `strfromf()` with "%f" as the format string and positive or negative infinity results in "nan" being written to the output buffer (ran on x86_64 linux).

Tests cover only `strfromd` and `strfroml` in [StrfromTest.h](https://github.com/llvm/llvm-project/blob/main/libc/test/src/stdlib/StrfromTest.h#L122-L127).

Start with the strfromf entry point and inspect libc/test/src/stdlib/StrfromTest.h, especially the existing strfromd and strfroml coverage. Reproduce the %f case for positive and negative infinity, then add equivalent strfromf coverage and ensure the output is no longer "nan".

Written by the indexing model from the issue text.