[libc][NFC] Fix mixed up biased/unbiased exponent (#75037)

According to [wikipedia](https://en.wikipedia.org/wiki/Exponent_bias)
the "biased exponent" is the encoded form that is always positive
whereas the unbiased form is the actual "real" exponent that can be
positive or negative.
`FPBits` seems to be using `unbiased_exponent` to describe the encoded
form (unsigned). This patch simply use `biased` instead of `unbiased`.