[libc] Simplify decimalStringToFloat and hexadecimalStringToFloat and improve their performance.

Combine two loops in decimalStringToFloat and hexadecimalStringToFloat that extract the digits and re-arrange them a little bit.  This slightly improves the performance of strtof and strtod:
Running libc_str_to_float_comparison_test parse-number-fxx-test_data/data/* on my machine (Ryzen 1700)
- with glibc: ~1.92 seconds
- with current implementation: ~1.78 seconds
- with this change: ~1.67 seconds

Differential Revision: https://reviews.llvm.org/D113681