[libc] Add __isoc99_fscanf alias. (#213125)

- Add LLVM_LIBC_ADD_FUNCTION_C_ALIAS macro to add another C alias public
symbol to a function.
- Add LIBC_CONF_SCANF_PROVIDE_ISOC99_ALIASES config
- Add __isoc99_fscanf for generic fscanf target if
LIBC_CONF_SCANF_PROVIDE_ISOC99_ALIASES is set.
- Similarly: scanf, vfscanf, vscanf.

GitOrigin-RevId: e4693c0579ed0f3df45a06b918d448795b738a58