[libc] LIBC_CONF_PRINTF_DISABLE_FLOAT behavior

When LIBC_CONF_PRINTF_DISABLE_FLOAT is set in libc config, if the user code uses float format strings currently they are treated by libc as regular string and gets printed to console. This is problematic change in behavior without warning the user. It will be preferable to emit a warning or provide any other means for users to be able to catch such instances in their code bases.

Start by locating the libc printf implementation and the handling of LIBC_CONF_PRINTF_DISABLE_FLOAT. Trace how float format strings are treated when the configuration is enabled, then review nearby diagnostics and tests. Done means users have a way to detect these format strings, with regression coverage for the configured behavior.

Written by the indexing model from the issue text.